Disassemble, clean, lube and reassemble. Sounds like the hammer isn't resetting when you shoot so it's either a displaced spring, dirt trapping it or broken altogether. There are loads of disassembly guides online so take a couple of hours and have a look at it for yourself - identifying the broken part should be simple once you compare it to that from a video.
Other than that you could send it off, but either way getting to the hammer in any of the MP7s requires significant disassembly. It's not hard if you're patient, but will be a little frustrating the first time.