iMac wont turn on

Ok so a few months ago, my iMac started lagging and auto shutdown when it heat up. One day, it wont turn on and it will end up with a crossed out circle after trying to start and asking me to press any key to restart. So it will basically keep repeating this. I tried everything including trying to reinstall and checking the disk in recovery mode. One day, I managed to reset the nvram and it worked fine.

Last week, my iMac died again and it wont boot up even after resetting nvram, and smc. So i decided to try and reinstall the software but it failed and now it wont start up except to disk startup but i cant choose to re-install macOS even.

What can I do?