Pc seems to power up but screen wont come on

Hello, I recently started having the problem where my comp. would shut off and restart on its on. I came to the deduction that my power supply ws goin bad. I ordered another one that ws defective and am waiting on the new one, but now my old supply will only power the cpu, but not my screen. when i hit the button the standby and green light come one,the fans start, my dvd and cd rom drives wrk and even an external cd drive i plugged into the comp wrked but the monitor light stays in standby(amber lookin) and never comes on...i am hopin that it is the power supply and that when i get it hopefully monday that all will be well but just in case, what other possibilites could it be so that i can be prepared for what i may need to do if the power supply doesnt fix this problem.....please help me