Define "fighting back". If you mean they will attempt to defend their land and cities, then yes. If you mean strike back at Russia then no chance. Ukraine simply doesn't have the tools necessary to mount any sort of counter-attack and even if they did the Russians effectively have them surrounded right now. So it would have to be ICBMs or similar long range weapons which they no longer possess.

Unfortunately this is what happens when countries are pressured to de-nuclearize, promises of Western protection and then fall to bullies.