Bass Problems

You may have broken a wire on you output jack while adjusting it. Another possibility is a short in your pre-amp. If so you would need to take it to a place that repairs guitars, ask around in music stores or a guitar center if your near one. I actually got my four string back from the shop last week for something very similar to this. They had it for six days but it only cost $30.

And less likely, the amp. Try changing out each component in the chain (bass, cable, amp) and see what changes. That will help you pinpoint the problem (but it is likely the output jack or the wiring for it).

“I bought my bass used”

Ok, what kind of bass is it?
Passive or Active, Pre-band 2 or 3?

“When plugged in to an amp, it was fine, but would occasionally come loose.”

What is coming loose, the cable connected to the guitar or the amp or what?

“The amp would hum when turned on, and get louder when turned up, but the bass wouldn't do anything.”

Amp hums - it's called “feedback” - a dirty, dusty bass can cause this as well as a loose/broken wire(s), bad/improper soldering, bad or soon to be bad pots and possibly a bad instrument cable.
Bass wouldn't do anything? As in, plucking the strings and no notes being sent to the Amp?

Save yourself some money, a Luthier should be your last resort. I look at it this way - if someone is going to take the time to learn the bass, they should find the time to learn how to fix and repair their equipment because a Luthier may not always be around when you desperately need one. Find a schematic / wiring diagram of your bass online. See if you can diagnose your own problem, and fix it.

There are more than a handful of people here willing to help others with issues.
But……please be precise as possible on issues or problems, otherwise you are not allowing us to help you in the manner that you need.
I don't know what kind of bass you have, though passive and active bass's are similar, they are not built and wired the same way.

Question: When you plug the instrument cable into our bass, do you have a tight/snug fit?

It's very usually just a wire on the output jack. You can unscrew it, pull it out and see for yourself if any wire is loose. If it's that, it's really easy to solder it back on.

Before trying any of these other options and taking the bass apart, first try cleaning the bass's jack port using a Q-Tip dabbed with rubbing alcohol. It could just be a matter of a little smear of dust or gunk inside the port. Also be sure to clean the plugs on the cable and the amp's jack port, just to be thorough.