The conventional wiring for a Hobbs meter is thru an oil pressure switch, thru a fuse, direct to the battery (so clever renters cannot fly around "for free" by turning off the master.)
1. Find the fuse, confirm that it's good. If no, fix the fuse (first look for any shorts).
2. Find the oil pressure switch, jumper the connections, see if the Hobbs runs. If yes, see if you can fix the oil pressure switch* or else buy a new one.
If no, the clock is no good. Maybe fix, probably replace.
*Edit: in step 2 check that there are no blockages in the oil line feeding the switch, if it appears to not be working.