Nikon d40x flash problems?

I was wondering if anyone can help me please. I'm having problems with flash on my Nikon D40x.
I have an SB900 flash and suddenly on auto and TTL mode my images started coming out really dark. I tested with the on camera flash and it was also dim, barely making a difference at all so it must be to do with the camera. I have checked all my camera settings over and over again and they are all the same as I always use in that situation (I do nightclub photography). I have a high ISO, slow shutter speed and maximum aperture, the same settings I always use in the nightclub with flash on TTL mode.

If I put the SB900 on manual mode I can get the flash just about bright enough but as I have to move quickly around pubs and nightclubs photographing groups of people I don't have time to keep changing my settings on the flash as well as the camera. I can't find any info on the internet about someone having the same problem.
Any help would be hugely appreciated!
Thanks

Added (1). Hello, thanks for your response.
Exposure compensation and flash compensation are both at 0. I've changed the batteries in the flash and tried a different battery in the camera too and given the hotshoe a wipe in case of dust. Everything is exactly how is usually is, I don't understand! It seems as if there's a problem inside the camera or something but after lots of internet searching I haven't come across anyone else who has had the same problem.

Without flash, does your camera meter properly?
If it does, be sure you aren't applying flash exposure compensation. This can be set on you flash and your camera.

Also be sure you flash is putting out enough light.

Looking at your original question and the additional info, I think it may be useful to do a little deeper detective work.

Start by running the camera without the SB900, and instead use the pop up flash. Check if that is underexposing, if it is then I'm afraid ther is probably something up with your cameras flash metering system.

Next try the SB900 in manual mode, at full power and see if you're getting the range you should be getting. If its not reaching the necessary range, then you are probably looking a flash problem.

I'm afraid that with a camera of the D40x's age, if it is a camera issue a repair may be more costly than its value.