I am a vocalist for a band and we use 2 different tunings, so my bassist ends up using my bass for another tuning until he finds a second bass. Last practice when plugging it in it made a very loud buzzing noise and nothing else; I tried turning all the knobs but nothing changed. So i took the back plate and the part to plug the cable in and it seems like one of the wires came undone from the plug in part but I'm not certain.
It sounds like the issue is the disconnected wire. Depending on which wire it is, it could be the ground wire, or one of the connections that actually sends the signal. From your description, it sounds less like it is a grounding wire that has disconnected, as you say the only noise that comes through is the buzz. Typically a grounding issue will still send the signal but you will have a buzz whenever all of the strings aren't being touched, where your hand is grounding it. As Tim suggests, you should be able to fix the problem by soldering the wire back. If you're not really sure about doing that yourself, it should be a relatively cheap fix by a guitar tech. You could also look for an instructional video online that will spell everything out for you and potentially confirm what your issue is.
The 'how' of this issue seems important to me. A wire typically isn't going to come undone without some sort of interaction. I've had problems with my input before, which were basically the result of stepping on my cord while it is plugged in, pulling the chord to the side and bending the internal workings, which loosened my connection and caused excess noise. This could be what happened to your bass but I couldn't be sure. If the internal components have been bent too much, it could cause the issue you are describing without a wire being disconnected. I have been running my cord between my strap and the body of my bass before plugging in and it has made a big difference, so that may be something to try out if the cord is getting stepped on a lot.
It's likely that the ground wire that goes to the bridge, and grounds the strings, has broken.
If there's no actual bass signal whatsoever, and it's all just 60 cycle buzz, then you've got a dead short between your hot wire and your ground. Likely at the output jack.
by the way...YOU DID change out the cable before you posted and made sure it wasn't a bad cable, right?
This is could be a problem with the pots. I had a similar problem before and removed the scratch plate (or like yours the back plate) and have all the pots changed. I decided to replace all the pots (and not just the one causing the problem) as it was a very old bass and it was likely that the other old pots would sooner or later cause problems as well. After replacing the pots the buzzing was resolved. Before it would make buzzing noise even on low volume, afterwards there was no buzzing.