Water not running in walk in shower the fall of pipe is fine

If your shower waste pipe is a stand along and drops into the hopper outside *which it should be) then an easy fix. If connected to a matrix of other waste pipes. basin and bath and then .... first eliminate is your basin runs away and your bath too as it used to, this will give an idea of where a block may be. If your shower is a stand-alone waste pipe, then as other members have stated, check for blocks in the shower trap (do not prod about too hard) then jet flush it through with the shower hose even it it does not reach the water will (unscrew the shower head first) Check how the waste pipe falls out side and if any right angle bends in the pipe that may be block stuff. Eliminate obvious block areas. Then call a Plumber if all fails - Good luck

check the trap is not blocked

The waste pipe or trap sounds like its partially blocked

No this answer is wrong I am a drainage engineer who deals with blockages, this is a typical plumbers answer, if the shower is not running away the shower waste pipe is blocked, you can attempt to clear the blockage using the shower head but this most probably will not work, ideally you need a drainage engineer to attend site and machine through the waste water pipe to clear the blockage most likely to be caused by hair.

Showers, Pipes, Traps, usually blocked, with Hair and Soap Scum, get Powerful Drain Cleaner, fill the Shower Tray and leave overnight, this usually works. If not get a Plunger from DIY Store, put water in the Shower Tray, operate the Plunger over the Waste outlet.