I haven't used MRP in the past, but this week I've been trying it out. It seemed to work fine for a couple days, it would take maybe 5 to 10 minutes and I'd get anticipated results. The last three times I've run it however, when it gets to Generating Material Requirements, BOM analysis: LABOR (the item we use to record time against WOs) it hangs there, even overnight.
 
I'd been running it with No Forecasts, No S-Type WOs, only the required Item Types and No Reorder Levels. I did just try it again with all the defaults and get the same result.
 
Any ideas?

Funny enough, I had just run SM-J-C this morning as part of my normal monthly routine, so I tried again but no change.

Check SM-J-Q for a loop someplace in a BOM.