Monitor not working

Hello,
ive just built a new computer for my brother but now has stopped working, was working for two or three days but now isnt. on the screen all i can get is no signal i have tried the monitor on my computer and its working fine. i was thinking about the graphics card but when i tried it on my pc same thing it works but on his pc not working at all then what is the problem ive checked almost everything but still nothing.
can someone help me please?