This could be caused by several issues, a loose cable, a faulty pc to monitor cable, faulty motherboard/connection in your PC or a faulty monitor.
We can check the cable as well as try a second monitor for you.
We can supply a replacement cable for about £5.
If your monitor is faulty and you wish to obtain your own replacement, we can arrange the loan of a 2nd hand monitor to tide you over.
If you wish us to obtain you a new monitor just ask, there are 100's out there and prices change daily.
If the video chip on the PC is faulty, it is probably cheaper to obtain a plugin card than have your computer repaired, these cards often offer better graphic displays than the default motherboard supplied chips.