Ok, first remove and reconnect the cable from the PC to the monitor and try again.
Second switch off PC and open the case, if you have a seperate graphics card carefully remove it, clean the edge connector with a pencil rubber and reseat it. Connect the monitor and try again.
If you do not have a seperate graphics card or if the above does not fix it then off with the power again and open the case. Remove and reseat the memory and all plugs you can find and try again.
If you are not comfortable doing this get someone to assist.
After all the above you might be looking at a corrupt file stopping the PC booting or a hardware fault.