Big Dog Rex completely given out on the left side.

Hey there, I was hoping someone could give me some advice Google is just not helping with. I'm not great with mechanical stuff so maybe I'm missing something obvious.

My big dog Rex zero turn has completely lost all movement out of the left arm. The hydraulic belt is still tight, and the owners manual says the hydraulics in this mower are self contained and should last the lifetime of the mower. I was wondering if anyone had ran into problems like this and what could be the root cause of it? Thanks