Did you try reinserting the cartridge and powering back up? My Genesis II is very finicky with cartridge insertion, just like our older Intellivision console which is where I learned about this trick. Just keep trying to reinsert the cartridge and do it FIRMLY, but not so much you break something.
I have a feeling I need to recondition the pin connectors on the console itself. The process seems easy but I am so fearful of ruining the console. Others have claimed that it fixed their problems with game power up on the first try.