iMac stuck on boot screen

Hello

So I have a 2011 iMac 27inch

Just upgraded software to latest version.

Seemed to then overheat to the point that the screen was garbled. So cleaned vents now it is stuck on boot screen and cycles round on that constantly restarting.

I have tried to get into all the recovery or safe modes and none work. Nothing seems to work. Potentially has a cable come unplugged etc as it won't even let me go into safe mode.