Where's my available balance on my home loan?

It took me 20 minutes of being on hold to finally speak to someone to query why my available balance on my home loan has suddenly gone to zero, despite there being regular payments and extra payments into that bond for years. And now I see that it is still showing as zero on the app. Now I need to access those funds and I'm unable to. Nedbank customer service agents are lovely but it is far too difficult to get hold of a human and nobody can tell me why the money has suddenly disappeared.