First I would want to think carefully if there was anything that might have changed the waterflow. Even if we didn't mean to do it! No start at draining tanks and then change of mind or anything odd like that? Easy way out would be to remember something we do and change it back!
But if that doesn't work out, some more looking is needed. So can you access this drawing of your plumbing and parts?
Looking down to page 137 will give the basic layout of the water lines, to give a better idea of where something might be stopping flow.
If you pull that drawing, I would start like this:
With water coming in on hose at item 1, verify cold water is flowing good at the cold shower faucet. That is the first connection toward the heater with a simple tee fitting to go on to the heater, so if water good on cold, check hot water flow there, as it is just a short trip to heater, through it, and back to hot at shower.
If no hot there, move to the heater temperature and pressure relief valve to open it and verify cold water is getting to the tank. Watch not to burn if the heater is on and hot water comes out of the relief!!!! Maybe open it first with water hose turned off and then open hose after relief valve is open to verify good flow!
No flow? The block is between the shower and heater but if good flow on cold, check for a blockage at the output on the hot line or along the way back to the shower as it is a short run between those points.
If unable to access the drawing, I can cut and copy it but reading the original is far better!
do a little checking for how it goes?