Apple iMac

Have an os x 9 something and had a black out then came on then off again for 5 1/2 hours. It was not on a power surge board and the main auto switch on main house bord did not switch off. It now wont switch on. I forgot to turn off the power at the wall…. bad not to think. Does anyone know if there is a way to reset computer so it will come on Power is available but wont switch on. It is a white apple with the round computer bottom. I thankyou in advance for any help I can get as this is a loaner computer and so need to have it fixed asap, Magieme