From your post, I don't know if I'm to assume you PERMANENTLY lost water pressure or just temporarily lost water pressure or lost water all together. SO, I'll address each one.
If you permanently lost water pressure, then you need to find out what other 'things' are connected to the same line. One of those 'things' is drawing water on a PERMANENT basis that didn't before. Is there a garden hose left on? Did you have something new installed recently?
If you temporarily lose water pressure, it's usually due to another 'thing' on the same line that's drawing water away on an intermittent basis. It could be person-induced (like someone running the dishwasher), or non-person induced (like the sprinkler system coming on for 15 minutes.) Again, find out what other 'things' are on the water line going to your tub to see if any of them are the cause.
If you lost water to the shower all together, I doubt the line is clogged. Check to see if someone turned off the shut-off valve anywhere along the line going to the shower. There may be more than 1 shut-off so, again, trace the line, ALL the way back to the source where it comes into the house, to see if any of the shut-offs are partially or totally turned off. Do be careful. If you have old gate valves, they may not work too well, and you may have a hard time determining if the valve is ON or OFF because you can't turn it. Hopefully, you can locate the MAIN shutoff valve before you start because if you spring a leak or break a valve in this process, you'll need to know where the MAIN valve is. First, I'd turn that off and test 'something' to see that you can successfully shut off the house water at the MAIN. If that's OK, turn it back on, and start tracing the line and check all the shutoffs.
Now that I think of it, for all intensive purposes, check ALL these things because you might find your problem. If you need more, post up. My best to ya and hope this helps.