The director of the power company, Vsevolod Kovalchuk, told Defense One that he is 99 percent sure a deliberate attack caused the outage. The event is similar to another blackout last year that was reportedly pulled off by Russian hackers, Sandworm. So far there's no direct connection between the hackers and the Russian military.
Meanwhile it looks like an app built to help quickly target the D-30 howitzers used by the Ukrainian military was hijacked with malware that could have potentially shared the location of those large guns with Russia. Security research firm CrowdStrike notes that from late 2014 thru 2016, a compromised version of the app was distributed through military forums.
The malware was reportedly created by Russian hacking group Fancy Bear which is believed to be behind the Podestra email leaks and has claimed responsibility for leaking the medical records of Olympic athletes.