iMac is turning on but a few seconds later just turned off by itself

I have an iMac (Mid 2011) and a couple weeks ago it continiously turned off (restart) while I was working on it or while it was on sleeping mode. But since yesterday I tried to turn it on but a few seconds later just turned off by itself. The display just shows a white screen with apple logo.

If someone knows how to resolve this issue please let me know.

Thank you!