It might have been a kindness for Varian to have remained mired in that depression. But he had a son to raise and a kingdom to rule. Slowly, he pulled himself out of that black despair and began to face his responsibilities as king. It was not made easier by the events of the Third War. While the destruction mercifully avoided Stormwind, a kingdom that had barely managed to rebuild itself following the end of the First War and the orc incursions of Ner'zhul's Horde in the Second, Varian was aghast to discover that one of the three father figures of his youth lie dead. Worse still, the identity of Terenas' killer was Varian's childhood friend and surrogate brother, Arthas himself. This was fairly beyond Varian's ability to even comprehend. Grand Lordaeron, the city he spent the years of the Second War growing up in, transformed into a shambling ruin of wailing corpses and unquiet dead. Terenas, who lent Stormwind a helping hand and gave Varian his kingdom back, killed by his own son.
To Varian, who had his father stolen from him when he was a child, the idea that Arthas would not only throw away everything Varian had ever wanted but would actively destroy it was madness. It was scarcely conceivable. Only the brute fact of its having happened, and so quickly that demons and corpses were capering across the landscape of the greatest of the original nations founded by the humans of Arathor within weeks of Dalaran's fall, lent it any credence. Refugees poured south this time, and Varian received them in imitation of what Terenas had once done for the people of Stormwind. But there was nothing he could do to restore their kingdom, destroyed by the hand of the one who should have saved it.
Following the Third War and the destruction of Admiral Daelin Proudmoore's fleet in Theramore, Varian found himself effectively the only reigning monarch of a human nation. Alterac, its ruling house dispossessed during the Second War, had been fragmented by the lack of a centralized leader. Stromgarde, land of the Trollbane dynasty, had suffered from the orc Thrall's liberation campaign and ironically from the Syndicate, founded by the very Perenolde family who had once ruled in Alterac. Gilneas had shut itself up behind a wall a decade earlier; Lordaeron was gone, and Dalaran as well. Theramore, the human settlement on the shores of Kalimdor lead by Jaina Proudmore, had proven entirely unable to defend itself if the Horde founded by Thrall decided to invade it.
In the end, Varian was the only king left.
When Jaina Proudmoore contacted Varian to discuss peace talks with the Horde, amazingly, Varian agreed to participate. Mostly it was his son, now grown to the start of adolescence, who convinced him. Truly, there was no one else to do it but him -- and just as clearly, as king, he must work for the well-being of his people despite everything that had happened to him and everything he'd lost. Unfortunately, Varian had no idea of Lady Prestor's role in his wife's death or the formation of the Defias; as such, he unwittingly fell right into the hands of her servants by agreeing to the peace talks. King Varian disappeared. Few knew his fate, although signs pointed to the Defias Brotherhood's involvement in his disappearance.
When a wandering amnesiac washed up onto the shores of Durotar and found himself captured by an orc shaman named Rehgar Earthfury, no one connected him with the missing king of Stormwind. Why would they? Sure, the human proved capable enough in combat to kill a crocolisk with a piece of wood. Absolutely, he killed an orc blademaster in single combat and claimed the man's twin swords. And yes, with his leadership and combat acumen, the orc shaman's team of gladiators (the druid Broll Bearmantle and the rogue Valeera Sanguinar) successfully won a championship in the ruins of Dire Maul. The human became known as Lo'Gosh, the Ghost Wolf, in tribute to his ferocity -- but even so, all that could be determined was that he could not remember who he was.
Escaping alongside Broll from Thunder Bluff, thanks to a feather granted to them by Hamuul Runetotem (perhaps with Rehgar's silent collusion), the man named Lo'Gosh slowly came to know himself. Arriving in Theramore, Jaina Proudmoore soon convinced him that he was more than he had dared believe himself to be, that he was in fact the lost king who had been on his way to her island for the peace conference. Dark magic had somehow stolen his memory, and a chance encounter with a naga band that attacked the ship Lo'Gosh and his allies were taking to the Eastern Kingdoms helped restore part of his memory.
I am the king of Stormwind, and no one will keep me from my people
While he still felt and acted as Lo'Gosh, a gladiator, he now believed his duty was to return to his kingdom. Encounters with a dwarf, Thargas Anvilmar, and the local Defias thugs in Menethil Harbor soon led to thwarting a Dark Iron attempt to destroy the Thandor Span. Sadly, Varian's old arms instructor, Thargas' brother Hjalmar, died during the attack. This led to King Magni Bronzebeard himself taking Varian to Ironforge, where he learned that there was an impostor sitting on the throne of Stormwind, claiming to be Varian Wrynn.
An excursion into the Dark Iron stronghold netted them part of the truth, as Varian's old friend Marshall Reginald Windsor was being held captive there with information on what was going on behind the scenes. Varian and his companions rescued Windsor, despite the man's regrettable tendency to run straight into packs of enemies without any regard for his own safety (trust me, Windsor was the original "Hey, these guys might have ice cream!" escort mob), and soon discovered the truth. It was the black dragon broodmother Onyxia who had wormed her way into power in Stormwind, and it had been at her instigation that the House of Nobles had cheated the stonemasons and caused the Defias to exist. It had been her will that Tiffin Wrynn should die, and it had been her claws behind Varian's kidnapping.
Armed with such information and backed by his allies in Theramore and Ironforge, Varian made ready to storm his city and reclaim his kingdom.
Sadly, things would not be so simple as that.