Probably the wire come off the oil pressure sensor mate.
It gets earthed through the switch when the ignition is switched on therefore illuminates the lamp. If the wire has come off the lamp wont illuminate.
Simply locate the oil pressure sensor on the side of the main engine block and check if the wire is still on ?
If it is you may have trapped it under the cradle when refitting it.
The sensor looks like this but may have a threaded connector on it ?