|Why did you post a hardware question in the gaming forum? Hopefully it will be moved because gaming isn't nearly as active as hardware.|
As you said, the problem isn't with the monitor, it's with the PC. You didn't post a make/model or any specs so it's difficult to suggest what to try. Were you having any issues before the monitor stops receiving a signal? Did you make any changes inside the case? When you attempt to power up the tower, are there any beeps codes? any lights? any fan noise?
If you list the system specs, please be very specific. Make/models where possible, especially for the power supply. Just stating the wattage isn't very useful.