The problem is with the pipe.
First, check whether the water outlet pipe (hot water pipe, water tank outlet) is clogged with scale.
Second, check the water mixer to see if it is broken and cannot drain water.
Third, check the shower head. It is best to take it apart to see if the shower head is blocked by scale.
Fourth, check the section of pipe from the corrugated pipe to the shower head to see if the rubber or silicone pad is twisted and blocking the outlet.