External water tank not filling after Irish Water cut water supply and re-enabled

Irish Water have lately been cutting off the water supply in our estate without any advance notice... latest instance happened this morning while I was in mid-shower and suddenly found myself sans water covered in suds.

Apparently they are allowed to do this if the leak is an "emergency"

Anyway, my setup is that I have an external water tank which then shoots water up to my bathroom/toilet via a water pump. The external tank is filled up from the mains.

Irish Water have reactivated the mains, but my water tank doesn't appear to be filling up again... it's likely an air bubble got into the system at some point.

How should I resolve this?