No hot water after replacing valve taps

The other day I replaced a leaking shower mixer cartridge. To do this I turned the water off at the street then turned a valve off under the hot water cylinder. Fixed the leaking mixer, but when I went under the hot water cylinder to turn the water back on, noticed the drain valve and what I assumed was an isolation valve were both leaking.

Then yesterday I connected a garden hose to the bottom cylinder pipework, turned water off at the street, drained the cylinder then replaced the two 3/4" valves. See picture. Turned mains water back on but no water coming out the hot water taps? Cold works fine! Any thoughts much appreciated!

https://trademe.tmcdn.co.nz/photoserver/full/1510199386.jpg

The pipework coming from the left side is from under the cylinder, and then it goes off into the attic space. The heating element had been turned off while dabbling around.

Assuming it's a low pressure cylinder and that's the only pipework under it, the pipe and tap on the right are probably the water supply into the cylinder from a header tank in the attic.
So you would've drained both the cylinder and header tank. So maybe get up in the attic and see what's happening in the header tank. Maybe some debris or dead rodent has blocked the outlet to the cylinder when you drained it, or the inlet float valve is playing up. In future, jam something in the header tank outlet to prevent it draining.

OK thanks for the thoughts. header tank is clear and seams to be working correctly. drained both tanks again with all hot taps open, refilled still no luck. Ended up turning a mixer tap on half and half then held the palm of my hand over the outlet, forcing cold water though the hot water plumbing, did this several times over a minute or so then water started flowing though the hot taps. Lots of bubbles for a minute or two then ran clear. Ran all hot taps for a while to clear everything. All working well now. Air lock?

Yes. Air lock.