Gateways show boards automagically. If you know your board is in slot 4 of gateway 5 and list conf med 5 shows no board in slot 4, log in to your media gateway.
show faults
show mm
in the output of show mm, you'll probably see 'initializing' or 'insertion failure' - or possibly no board.
reset mm v4 will hard reset the slot. Try the module in another slot, validate any other module works in slot 4, and resign yourself to knowing you have a defective module or slot!