Monitor shows 'no signal', why?

This guy I know got a new Internet/phone/cable TV connection.

(Which new company I do not know)

Before he changed service provider his computer was working fine.

Now when everything is turned on, the monitor shows a little window

which says..."no cable signal'....I checked for him to make sure all connections

were properly placed...they all seemed to be...then for the hell of it, I switched

the electrical plugs in the electrical outlet....I KNEW THIS WOULD MAKE NO SENSE,

since both the computer and the monitor showed that they were receiving power.

But I did it anyway....and for a moment the monitor displayed the desk top!!

Then using the mouse I clicked on the Explorer browser,

and the monitor went blank again...showing that little window

which said "no cable signal."

What could possible be wrong with the computer?

Some guy said it could be the "card" in the computer tower

where the monitor cable attaches to....that the "card" is faulty or lose??

I know almost next to nothing about computer hardware and software.

So if you think you know what might be the problem, please explain it

as if you were "talking" to a computer nincompoop...because you are...;-)

Thanks one and all