Either the flex plate went again or the hyd pump is not pumping oil. Fill the reservoir to the proper level, remove the screen in the standpipe and clean. Check if the hydraulics work.
If not, remove the hydraulic pump, leave the throttle lever in the off position and crank the engine to see if the shaft in the hydrastat pump is turning. If not the flex plate failed.
If it turns then the hydraulic pump is the problem. It is possible, if the oil level is low, the pump picked up some air and will not prime itself back up. If that is the case you should disassemble the pump and check for wear.
If all appears to be ok install a new seal kit. A new seal kit is necessary since the seals are not applying enough pressure against the gear wear plate. When this happens the pump does not generate any suction. As long as the pump is primed it will work but if air enters the pump it will not pick the oil back up.
