Basic questions perhaps but try -

Is it switched on?  Is there any sound or lights on?  If not, try a switch
at the back (on the power unit)  Not all computers have one but I have had a
call out for a computer that did not start.  It turned out that in moving
the computer a finger had accidentally turned it off.

Similarly make sure the monitor cable is secure in the socket.

Is the monitor switched on?  If you can hear the fans as but there is no
image on the monitor, it could be either the graphics card has failed (easy
and cheap to fix) or the monitor has failed (easy but not so cheap to fix).

Regards.

Bill Ridgeway
Computer Solutions