i suppose you have already checked the battery?
take a voltage reading (no load)
then activate the hydraulics and take a reading and see what it drops to.
On a 36v system it should read about 37.5 to 39v fully charged, and when loaded it shouldn't drop below 36v.

now there are some other possibilities that can cause no/slow drive without displaying codes like:
-bad directional contacts
-faulty power cable
-partially blown power circuit fuse

just some things to check for, there may be some other things some others here may know and post about but these are things off the top of my head that come to mind.

good luck