Do you have normal water pressure elswhere in the house? A crude test is to turn the cold tap on just enough to produce a trickle of water then try to stop the flow by putting your hand or thumb over the outlet. If you cannot stop the flow then your water pressure is normal and your water flow problem is due to a restriction in the plumbing.
If it's sudden, I had a similar issue with an airlock in the system, something to do with my water tank being rubbish, it does it every time I try to run a bath. The lovely people on here helped me fix the airlock by turning on the hot water tap then blocking the tap with my hand and running the cold water so it runs up the hot water pipe and clears the airlock.
If you don't do it right though, it can be messy...I got rather soaked before I sussed it :)
that is what happened to me last week when kitchen fitted. pedestal sink just giving me a trickle - guy went up put his thumb over the trickle and stopped it - then the next tap came on like new. try it
If a shower has low pressure but there is plenty of water coming out the tub spout or in other parts of the house the problem might be the shower clogged with hard water deposits. This can be resolved by replacing or cleaning the shower head. Unscrew the shower head and turn on the valve and divert it to the shower head to flush out any debris. If you have good pressure with the shower head off then you know the problem is within the shower head and it needs to be cleaned or replaced http://ibathtile.com