Monitor turn on first time only, next time it doesn't, monitor still working!!!! MO error? Help please

[nguyenminhtung]

here is my situation,
I build a computer and when I turn it on first time, everything fine, monitor flash and showing all information of motherboard as it usually does, after that I restart it to install OS, only the case working but the monitor doesn't show anything at all, only the green line flashing even I know for sure the monitor is still good. Do you know why? it happened to me alot, I used to unplug power cord and turn on again, but now it won't work