There is a filter of some sort in that canister. I suggest you take the filter out of the canister and see what condition it is in. I guess I would do that after you get it working again unless that doesn’t happen by priming the pump.
The line out of the top is the inlet pipe to the pump that
@TheOldHokie describes above. That would be my first thing to try (prime the pump). I assume you were running the tractor for some time until you noticed that it was not working. That may have damaged the pump but I am hopeful that the pump is just really dry and can’t draw enough fluid into it to get itself primed thoroughly through the filter.
Since you are unaware that is the filter I assume it may be pretty dirty. The illustrated parts list above looks like it is a screen that can be cleaned but I assume it will drain the whole transmission of fluid if you take it out.