Jim - first I don't much about plumbing, but I had a similar problem, and with the advice I received from this web site and with the leading of the Lord it was resolved.
My hot water in the bathroom would just trickle out of the facebowl and shower. One day the village turned the water off, and when they turned it back on, I did not have any hot water coming in the bathroom at all.
I replaced all the piping from the main water line to the bathroom but that did not work. What I failed to do was to check the 'riser' lines that run from the shutoff valve up to the faucet (or in your case I would check the valves also).
First tip - work your way back. Start from the faucet, remove the aerator/nozzle to make sure no deposits are blocking it.
2) Check the riser tubes. Turn off the water at the shut off valves under the sink. Remove the riser line if possible and blow through it, or run a hanger or wire through it to make sure it is clear. Replace the end of the tube that connects to the shut-off valve. Get a bucket or pot and hold the loose end of the tube in it as you turn the water back on. If water blasts through, you have possibly fixed your problem.
3) If the above does not work, it would appear you may have a clog before it gets to the shut-off valve.
If you can get some help, while you still have the riser tubes loose, turn off the water at the main shut-off valve coming into your house or apartment. Have your helper to turn the water back on as you check whether the valves are working or not.
In my situation, the piping goes from 3/4" piping down to 1/4" riser tubes. It does not take very much rust or deposits to clog those riser tubes.
I hope this helps. I am sure others on this site will be able to provide other helpful information.
: Previously, our bathroom sink had some pressure, but I didn't think enough. Though I don't know a thing about plumbing, I finally got around to seeing if I could increase it by tackling the only thing I could see, the two valves under the sink (for the hot and cold water). Since I didn't know if it was already at full blast or not(as the valve was old and hard to turn), I turned on the water from above, then started turning, which was closing it. I opened it back up to full (which it had apparently been at before). Then later that day I noticed that the pressure and the amount of water coming out was worse, much worse than before, now coming out in just a trickle. Any suggestions on how to get it back to at least what it was before, and better? Where'd I go wrong? Thanks