Check the following
1 Is the pc and monitor power cables connected? Do the lights come on, on the pc and monitor?

2 Has the fuse blown in the plug??

3 Is the plug socket working? Plug another device in that you know works

4 Is the monitor cable plugged into the pc correctly?? Re-seat if it looks okay

5 Is the PC beeping?? If yes look up what the beeps mean in your motherboard manual

6 Reseat the processor and memory

7 Serious problem, Most likely the motherboard has developed a fault and needs replacing.