|Are you connecting while the PC is running and the monitor is switched on?|
If so, DON'T
All connections should be done with power off. Please give a full description of the problem including when it first began. Do you have another monitor to try?
Have you isolated whether the problem is in the tower or the monitor?
Does your system have integrated graphics in the motherboard or a separate graphics card?
Goin' Fishin' (Some day)