iMac flickers when turned on

When I was playing something on my iMac last night I got thirsty so I went to go get something to drink and when I came back the screen was black. I thought the iMac just went to sleep so I tried to wake it up but it didn't work so I restarted the iMac and instead of a white screen when you turn it on the screen was full of flickering colors so I went on the Internet with a different device and looked for my problem but I couldn't find it. When I got up this morning to try turning it on again it started to flicker with a million colors like last night so I searched the internet again for the problem and I couldn't find it some I decided to ask on apple support.