My husband and son were both showering one morning when they heard a single metalic sound like a "clank" or "thunk" from somewhere. In the master bath my husband saw the water turn quite dark colored for a couple of seconds and then turn cold. My son in the hall bath saw a change in the water color as well before turning cold. On checking the two bathrooms we have discovered that the lavatories don't have hot water either. The basement half bath doesn't have hot water in the lavatory. Only the kitchen sink has hot water. The water heater is in the basement and is about 5 years old. We have had two plumbing companies out to look at the situation, and I can tell they are stumped. The plumber today said that he would start by taking off fixtures, looking at the cartridge etc... Somehow, I don't think taking off faucets is going to do more than run up my plumbing bill. It is an older home. Please help!

first thing that comes to my mind is a recirculating system that has developed a problem. BUT, i'm sure any plumber with half a brain would have looked for this.
one other idea would be a dip tube in the water heater has broke. it is a simple test to feel the pipes coming out of the water heater to see what happens when a faucet/shower is opened.

You go to the faucet that has hot water and then trace it back to where it connects to the pipe for the faucets that do NOT have any. Somewhere in that area is the problem. Possibly a broken valve.

We tried to take your advice this weekend. The water heater hot water outlet splits into two copper lines immediately and goes into the cement of the house foundation which is five feet off the floor here because the house is a split level. Then one line comes out in the wall behind the kitchen sink. So it seems that the separation is right at the water heater and each line runs through cement until the lines come out of the walls. The 3 bathrooms are all clustered together in one direction from the water heater, and the kitchen is in another direction. We can probably locate the bathroom line in the wall that separates the bathrooms by pulling off paneling or sheetrock to see what it looks like in there. Don't know what exactly to look for, though.

Thanks for the suggestion. We found out the water heater is much older than 5 years. More like 10 or 12 years old and no water softener. The water in our area is extremely hard too. How would we check out what you suggested?

Unfortunately, with copper piping, what you describe should not have happened. Therefore, it appears you have some unusual situation which probably requires an expert plumber to examine and diagnose what happened and determine how to cure it. I would NOT start taking walls apart until you know the answer, because even if you did do so, the cause may not be obvious.