Have you ever needed to take a shower under low pressure? It is not a very pleasant experience. It makes it take much longer to rinse off, and you might not feel as clean afterward. So, what could the cause be? It depends. In some cases, it could be due to calcium buildup in your pipes. With hard water, this is a possibility, and it is somewhat tricky to fix. Of course, getting a water softening system installed will keep the problem from getting worse, but cleaning out your pipes, will take some more work.
It is also possible that someone closed a valve partially, whether on purpose or by accident. Following the water supply back from your shower to the water heater could give you a clue.
Another thing to take a look at is the control for your shower. If this fails, it might be necessary to have them replaced to get back the water pressure to normal.