Monitor shutting off after BIOS

[RealTech]

i'm having a problem and i hope you guys can help me out. My monitor shuts off after windows logo, fan still runs and bios light stays on, hdd has no response. Monitor only stays on during bios operation. Tested two different hdd's same problem occurred. first we thought it was a power issuse and swapped out similar power supplies same result. we also applied thermal compound onto heat sink just incase it was an overheating issue and again same result. its a Dell Optiplex GX270 with all original components. Any tips or ideas will be greatly appreciated, thank you.

 

[RealTech]

Just to update... I was beginning to believe it was a hdd failure but after trying 3 different hdds i get the same result... I'm at a loss. Does anyone have a solution or should I just bury her and move on????