Hi my computer doesn't give signal through the monitor I have tried 2 computer can anybody help me?

And the other thing on my third computer the power isn't worker I have black small boxes which slide onto wires e.g.. Power sw

If anybody can help me I will be very grateful

You need to be more specific as to the problem.

The "black small boxes" are probably ferrites. Leave them on. They are used to attenuate RFI noise, In essence to "choke" the RFI from coming out of the case and possibly causing interference with other electrical devices such as wireless telphones, TV's, radios, etc.