|Could be anything at this stage;|
Do you get the initial system 'beep' as usual, or has this changed as well - either absent or multiple beeps?
Also does the monitor power LED show. Anything on the screen, such as a no signal message?
If you aren't sure, as a process of elimination, can you test the monitor on another PC, or borrow another monitor to try out.
Otherwise, check the usual;
plugs, sockets etc.