It has to be simpler than the weight of the hydraulic fluid. Stiffer shifting maybe, but no shifting at all seems over the top.
If the tractor sits outside, has been out in the rain, or has been washed recently then my guess would be that somewhere in the shift linkage there is water frozen.
When the shifting comes back does it come back fully functional or does it gradually loosen up as the tractor warms?
If the shifting comes back very slowly and loosens up gradually as the tractor warms then the hydraulic oil issue may be more likely.
If it comes back quickly and fully then the water (ice) conclusion is more likely.
Checking the hydraulic oil for water contamination may be another path to resolve the problem. The filter could be restricted with ice or the pump frozen.
Just for giggles, does the FEL (if equipped) and/or the 3PH operate normally when the shifting acts up? That result could go a long way toward diagnosis.
EDIT: Just noticed you have a L3650. Great little worker and mine was worked hard. Just sold it - it was getting a little long in the tooth at 1200 hrs. The clutch went out on the buyer at about 1250 hr.
