Hey guys

I have a 2010 iMac desktop that has been sitting around unused for a while. It stopped working about a year ago and I am keen now to get it fixed. It is having the same issues as before also so neglect might not be a factor. I installed the latest OS system in 2018 (cant remember what one unfortunately) which I realised after may have been a bad move as per other internet advice. And may be why i'm having issues now. I also only have a bluetooth keyboard and mouse for this computer if that makes difference. When I turn it on it goes to the grey screen and loading bar, it will load all the way then a 'prohibited' sign will pop up. I have done a heap of googling and tried solutions i've seen. Ive put it into that reboot loop for a few turns with no result. I've tried holding down things on it starting up like opt-cmd-r and shft-opt-cmnd-r, holding down c etc. Is there any way I can recover this iMac? Everything was backed up before I updated the OS so it's not a major concern if not but it would be great to have it back up and running.

Thank you!