Just changed the immersion on the cylinder, hot water pressure died?

Just changed the immersion on the cylinder, all appears to be working well, not tripping and been on for about an hour and a half now. I took about 10L out of the tank to fit the new immersion (after draining the loft tank completely and turning off the water inlet) however water pressure on the hot taps seems to have died completely. Any ideas on what to do or do I just need to wait?

Cheers