Failing power supply and hard drive are not uncommon for a 12 year old model.
Hi guys  A week ago my g/f's Optiplex 755 was working fine then when she pushed the startup button the amber flashing light came on so she waited for 10 to 15 minutes when her desktop appeared normally, later on she turned the pc off the next day the same thing happened but it took 25 to 30 minutes for the desktop to appear with the green startup button, the following day it took 45 minutes same thing happened after waiting while the amber light was still flashing the green light appeared once again so for the past few days shes been leaving the pc on and transferring all her files on on a removable disk. The number 2 light appears when shes on her desktop and the green startup button is green, she now fears if she turns it off the desktop it wont appear again so I cant tell ya what lights appears on her startup, she as no pci card installed and hasn't changed anything Hoping to find a solution to her problem  and that its not her motherboard ! Thank you Rick  
Optiplex 755 start up problem