Electrical system failure

The voltage being lower than 12v indicates you have a charging system problem. Typically, with the engine running that should be > 13.2 volts.  The stator has separate charging coils for the engine ignition system so you can have a failure of the airframe charging system and the engine will keep running as long as the engine driven mechanical fuel pump is working. The likely areas of failure are the alternator (stator), the voltage regulator, or the wiring between those two items. 

Regarding the complete loss of your electrical systems, it's possible that if you do have a master solenoid the battery voltage got low enough to go below the drop-out voltage of the solenoid. At that point the battery is disconnected, and assuming the alternator is not charging than your airframe electrical bus is dead. The battery may recover some on its own (without charging) which may explain why it started a couple hours later. With a lead acid battery, a standing voltage of 12.23 (with no load connected) is considered fully discharged. A fully charged battery will have a standing voltage of 12.89 or thereabouts.  What was the voltage of your battery when you tested it after the incident? I'm betting it was close to fully discharged.  

Also, this may have been going on since the work was completed on your aircraft sprag clutch. If your aircraft electrical loads are modest, then it may take a few hours of use before the battery gets low enough to drop out the electrical system completely. The first thing I would check is that everything got reconnected and look for any damage that may have happened during the earlier repair (I don't believe in coincidences of this nature).  After that you will need to test on the stator as outlined in the heavy maintenance manual. A master solenoid would not have a temporary failure, they are an electromagnetic device that ether works or not, a failure is typically immediate and permanent. 

A charging problem that I encountered several times (5 times!) was the regulator connector which had a problem with its "G" terminals, I removed the connector and installed separate insulated and good terminals. quality .
The check always begins by measuring the voltage on the two "G" wires of the regulator by rotating the motor at +/_ 3000 rpm, the voltage is around 20V AC or more (if I remember correctly!) This voltage increases when the engine runs faster. If this voltage is present, check the regulator, if it is defective, the "G" connection pins have heated up and have changed the color of the compound at the base of the regulator lugs. In all 5 cases that I encountered, the compound of the regulator connector, originally red, had become brown.
Swapping the connector with separate insulated terminals solved the problem.?

Jacques

Another thing, a very good ground connection of the regulator is very important.
A direct connection between the negative pole of the battery and the regulator ground is a good practice

Jacques

So a week ago my mechanic and I fixed the issue. My mechanic had identified a wiring harness that looked like it had a couple shorted wires. Apparently whoever built my plane joined a couple wires together with spade connectors and wrapped them with electrical tape. Over the years tape became brittle and was probably the result of the shorting. My mechanic fixed that correctly and we installed a new voltage regulator. Started no issue and the voltage output was normal. Test flying confirmed issue fixed. This is the second time I've had to replace the voltage regulator in 8 years/350 hours. Maybe I should keep a spare handy.