What should I check next?

Hello,
I'm doing my best to discover if my issue is a simple fix or if I need to call in help to fix it. The issue is such:

I got a new air conditioner, pretty strong ... lots of watts. It ran in my room for a week or so before all of a sudden the power went out on the circuit it was on. Nothing much else other than a CFL light bulb and a couple fans plugged in to the circuit. I checked the circuit breaker and it was not tripped. Reset it anyway and got no result. I checked for and there are no GFIC outlets on the circuit. I went ahead and made sure none of them on the other circuits were tripped. I then went and bought a little volt meter to check if any of the outlets were active according to that. I found no outlet on the circuit that would read as active. I double checked the meter with the other circuits and it went off immediately.

That's where I am so far. I think the next thing to determine is if the problem is at the breaker box maybe ? I'm not so sure. One other thing to note that seems odd to me is although my breaker box has written on it that the water heater is on a different circuit, if I shut off the circuit that is the issue the water heater does not work as well. May just be a mislabeled box but the circuit that is not working at any outlet whatsoever definitely needs to be on for the hot water heater to run.

Any suggestions on what to try next to locate the exact issue? Thanks in advance.

My first suggestion would be to flip all the breakers off and then on. When a breaker trips it doesn't go all the way to the off position, as you said the breakers may be mislabeled. Guess this is good as any for a starting place.

...oh, and welcome to House Repair Talk, somone with some actual electrical knowledge will be along shortly.

Isn't the water heater a 2-pole, 240Volt heater?
The A/C unit is 120Volt if it shares a circuit with light and a couple of fans.
Therefore, I would have to assume that the two problems are not related.
Sounds like you overloaded a 15-amp circuit with fans & A/C. You possibly burned up the neutral wire, which would explain why the breaker did not trip.
You'll need to get a positive ID on the circuit that the A/C is on and test hot to neutral and hot to ground in order to determine the cause of the problem.