I have a shower that my family uses every day. Today, I went to get in the shower, turned on the water and waited for it to get warm. The water never did get warm. I tuned off the cold water, left the hot on (it's a two handle shower) and the water coming out was still cold. I checked all the other faucets and we're getting hot water just fine everywhere else in the house. Like I said before, this wasn't a problem until today... any ideas?

That doesn't make much sense, if you have hot water in all the other fixtures you should have it in the shower. You have hot water in the sink in the same bathroom?

Not from the little bit you have told us.

If you have any info about the shower valve, it would make it easier to steer you in the right direction. It's most likely a stuck pressure balancer, but their position varies by manufacturer.The more you can provide the clearer your answers will be.

Two handle valves do not have a balancer, if by "two handle" they mean hot and cold, not a temperature and volume, two handles.

I missed that. It will require on-site inspection by someone that knows what they are doing.

turn the hot on in the shower. at the same time turn on the hot to the kitchen and any other fixtures. what happens?

You have to find where the cold water is crossing into the hot side and stop the cross connection. These can be tricky, because there often isn't a for sure try this, then that fool proof way of diagnosing this because every situation is different. The more information you post up here the better the chances will be one of use will figure it out online for you. But you might have to call a plumber...

Places to look:

Did anything else change in the plumbing system recently..

Single handle faucets and shower valves are common causes of this... To check if one of yours is doing this someplace else in your house: Turn on the hot water at the suspected fixture long enough to warm the angle stop under the sink and turn the water off. For single handle shower valves remove the cover plate and run the hot water till the hot pipe get hot and turn it off. Then run the shower getting cold on the hot side... Go back and feel the hot angle stop or pipe at the suspected fixture. If it's your cross connection the hot angle stop or pipe will cool from the cold water being drawn through it.

Do you have any other showers in your house with a hand held spray that turns off water flow while leaving the shower valve it's attached to turned on??

Those are my 3 best guesses withouty seeing the hose...

Packy may be onto something as well. About 2 weeks ago I was called out for same problem, hot water everywhere but one shower. Turns out the showerhead had some debris in it that was restricting the water just enough that it wouldn't allow the on demand hot water heater to fire. As soon as I took the head off the water heater fired instantly and the water was hot. A quick cleaning of the inlet screen on the showerhead and all was well again.

Having a tankless water heater, it that is what he has, would be one of those "missing" bits of information we could use for a diagnosis.