Back will run, but will not move forward or backwards.
What letter is it? K, L, or M? When you say "back will run" do you mean the engine?
If it has the button on the loader control for the clutch check it isn't shorted and engaging the clutch.
Could be shuttle relay and timer relay or the relief valve snap ring is broke (very common on the SL machines) and of course there is the FNR lever that could be bad.