So for about 2 months now i've owned a Audiotechnica AT2020, which has been working fine for the whole time i've owned it. It is hooked up to a Newer NW-100 power supply, and uses an xlr cable to get power to the mic, and a XLR to USB to help with getting the mic to work with my PC. Recently, i've been having an issue where the mic becomes really low in sound and my friends can hardly hear me. I have to move it and do other things before it works, but today it just started not working flat. I have to yell to even get any sort of sound out of it. Would love some help here, thank you.
As Tetsujin commented, you should test it with a different phantom power source and different cable to identify where the problem is. I'm rather doubtful it's the cable but I would test it anyway. Do make sure that you check it isn't a gain knob turned down, a pad button pressed or a dynamic processor (compressor etc.) on the channel.
Most likely, your power supply is failing. If you have a multimeter handy, you could test it to make sure it gives 48V (48V is the standard voltage for phantom power but do check the spec sheet of your power supply because I think some manufacturers can use as low as 5V).
If it doesn't, you can either get a new one or repair it. Repair-wise, I would change all the electrolytic capacitors and check the voltage regulator(s).
If it isn't the cable or power supply, I would open the microphone very carefully and give it a cleaning with a soft brush. Take pictures of the process to make sure you don't have any bits left when you put it back together. Although I've never looked into the electronics inside a microphone, I would have a look for obviously damaged components. Hopefully, it will be something you can spot with a naked eye.
P.S. I forgot to mention: do none of the above if your equipment is within warranty.