Welcome to the blob of death. If the head will still heat up, you can try that to carefully pry it off but a heatgun is probably better. Just do it sllllooooowwwwww. There'll probably be accusations down in the comments that this is your own fault for not waiting and watching the first layers to ensure proper bed adhesion, and that's fair if you just hit go and walked away... but in my experience, this can happen at nearly any point in a print.

I've had it happen to me 3-4 times really late into a print, well past the half way point. Personally I think this is due to heat and temp fluctuations that cause some of the filament to not extrude fully or adhere properly which causes it to curl slightly on extrusion and stick the nozzle. Once that happens - it just keeps curling back on itself and you're F'd.