The rover has been quiet since June 2018, when it reported an encroaching dust storm that obscured its solar panels and left it with dwindling power. NASA gave Opportunity 45 days to broadcast once the storm cleared, and stopped actively listening after that period. The administration made periodic attempts to communicate after that in hopes Opportunity just needed the dust to clear from its solar panels, but many now suspect that the vehicle may have suffered some kind of damage.
It's a sad end, but you can take comfort in knowing that Opportunity led a long and productive life. It landed just over 15 years ago on January 25th, 2004 for what was supposed to be a 90-day mission, but clearly exceeded those expectations. It helped reveal that Mars once had water oceans, and studied both the atmosphere and space from its rare vantage point. It even smashed an off-Earth driving record in 2014. Simply put, it's been an invaluable tool for understanding the Red Planet even though it has frequently had company from rovers like Spirit.
Thankfully, Mars exploration is far from over. NASA's InSight lander touched down in November to study the planet's geology, while rovers like ExoMars and Mars 2020 are close to launch. Opportunity played a historic role in space science, but it was one chapter in a much larger book.