Deb
Did anything else happen 4 weeks ago to coincide with the problem?
Never trust a plumber (or anyone else for that matter) who, when they cannot find the cause for a problem, suggests replacing things to "see if it fixed the problem" (and expensive parts at that). That is absolutely ludicrious. There is a cause, and it is something that you should be able to figure out. Very, very seldom are we unable to discover the cause of a plumbing problem after investigation.
What exactly did the other plumbers do? Did they disassemble a faucet? Did they check the condition of your water lines (what material are your water lines)? Did they offer any real possibilities (a clogged valve/nipple/union/etc at the water heater is not a REAL possibility because 1st floor would be affected, just to a slightly lesser degree)?
I would start by pulling the hot faucet stem/cartridge/disc and check for debris. Hold your hand over the hole and have someone else slowly turn on the shut off to see if you have water to this point and flush the line if there is. If there is debris in the lav, there is probably debris in the tub/shower valve. If there is no water, I just start to work backward--check the riser, check the stop. If the problem is in the lines in the walls/floors, you can back flush to the water heater. There are several ways to do this, the easiest depends on your individual plumbing. Basically you want to shut the electricity/gas and water off to the water heater, hook a hose to the drain and run outside, open the drain and drain the water heater down some. You want to connect a cold water line to a hot water line and turn the cold on to force it backward through the hot lines and into the water heater, hopefully pushing any debris back the way it came and into the water heater. You can disconnect the lav risers and run a ss braided supply line from the hot stop under the lav to the cold stop, turn both on and backflush. This is generally the easiest. But, you can backflush through the tub/shower valve, if it is a non pressure balance valve. To do this, you need to plug or cap the shower head (remove the head and put a 1/2" cap on the threads) and/or the tub spout and turn both hot and cold on. The tub spout is harder to plug off, but you can generally hold a rag tightly to the spout and get it to flow back.
Or, you could have old galvanized water lines that have internal buildup to the extent that the water supply is affected. There is really no repair for this, replacement is usually your only option. Age is not the major determinating factor in how long galvanized water lines will last, water quality is. I have seen galvanized that is 50 years old with virtually no buildup and 7 year old lines with less than the diameter of a pencil opening--water quality.
If you don't feel comfortable checking these things out, call another plumber. See if you can find a family member, friend, co-worker who can recommend a plumber. A tradesman who does good work at one job most likely does good work (period) and vice versa. This got longer than I intended, but it fries me when people hire a plumber(s) who doesn't do diddley, offers no REAL possible solutions, and tries to sell you something you might not need because they do not know what is wrong
So, I wanted to give you the best, most complete advise I could without being there. These are things that take average mechanical skills (except possibly disassembling the tub/shower valve if necessary) and do not take any expensive tools. You could need stem sockets, which are inexpensive.
However, a competent plumber should most likely be able to check all the stuff I mentioned in an hour or so depending on your system--do shut off valves shut off? How accessible is everything? Condition of existing plumbing? Etc. Shame on those plumbers that (IMHO) didn't do their jobs and/or tried to sell you something that they should have known could not have fixed the problem (and if they are a real unscroupulous type who did this because you are a woman, I don't even begin to have enough bad words to say about them!). There's an answer. You just need to check out all the possibles (the list isn't that long) and when you find it, fix it. I frequently tell people that they pay me just as much or more for what I know as what I do. Many repairs that service plumbers make can be done by a person with average mechanical skills. The problem is that most of them do not know what the problem is and if they do, they do not know the fix. Frequently the fix is something simple, the diagnosis the tough part. A plumber that cannot figure out the possibilities and eliminate them one by one is not much of a plumber.
Enjoy your day.
Deb
The Pipewench