Monitor switches on then goes standby

I’m trying to sort out a friend’s computer and i’m beat.
he’s got a BX board, P2 Celeron.Amibios.
On board AGP graphics, but recently installed Hercules Prophet 4000 xt 32mb Vid Card, which seems the obvious cause of the problem.
there is no display on the monitor, it just goes into standby, so there’s no way into bios.
Have tried all I can think of to isolate the problem.There is power to the board.
have tried plugging the monitor into both the original socket, and the new card. Tried with the new card out, still no joy.
All cabling is fine, tried a bios reset, even replaced the battery.
Any Ideas?