UGH!
This morning I plugged in my bass and turned on the amp, all ready for a nice productive practice day. Nothing came out of the amp. I switched to a different bass and nothing. I even tried my six string guitar… nothing. I tried different cables to see if that was the problem. I unplugged all the pedals. None of those things worked.
I then ran a cable from my laptop to the “aux in” jack on the amp and played some music. BOOM, it came of the amp speaker just fine. Then, I plugged my bass into my audio interface and connected the headphone jack directly to the amp. BOOM I could play my bass through this connection, but not the normal input jack on the amp. Obviously, there is a problem with that connection.
Fortunately, I’ve only had it 6 months, so it’s still under warranty. I’ll be making some phone calls tomorrow.
Interesting… I didn’t even think about that.
I just blew the jack with a compressed gas electronics duster and now it works. Can a speck of dust cause such a problem?
A speck of dust would be pushing it – but something more substantial (a stuff of dog hair, a strand of yarn, a fern leaf, a rat’s tail, a loaf of bread, a dove, half a camel) could easily do that.
Seeing as to how there was only one input not working, Joe’s suggestion made perfect sense!
Yeah, I’ve been wondering what happened to the other half of my camel. Now I know.
this is not exactly the same thing, but I will give you a little trick, very useful for old amps and music gear in general. often you can get a bit of corrosion on the contact parts inside a jack plug, resulting in a very high impedance (which means a very bad signal and often no sound at all = the plug just doesn’t work).
if you suspect a jack to have this problem, you can plug and unplug it 10,20, 50 times … the goal being to scratch the corrosion and bring back the bare metal. it’s very primitive but it works very often. worth a try if you don’t want to open the amp ! also very useful for old cheap amps (think Marshall Valvestate series : they all have this problem on the effect loop plugs) where the repair cost must be minimal.
When I was young (before the War), I found that spraying the jack with contact spray, and then inserting, and repeatedly twisting it once in in the socket, would often have a similar effect.
Could you be more specific about what you mean by “contact spray”.
Nope. That’s what it’s called. 
It’s a substance that removes corrosion from surfaces that are supposed to conduct electricity. It comes in a spray canister.
Be really, really careful spraying that stuff into an amp. Many of the sprays made for electronic contacts have alcohol bases (or other solvents) that are super flammable. Amps get hot. Bad combination.
If you are going to use it, instead I would actually spray a cable plug with it and then do what @terb suggested, insert it into the jack and remove it like 50 times.
That’s some good troubleshooting! 
…and that’s some funny shit, right there! 
My imagination may have gone a bit overboard…
I’d go as far as to say “NEVER EVER spray it directly into an amp. Do not allow the thought to cross your mind.”
That’s why I’d spray it onto the jack, and then insert the jack into the amp.
Yep. I’ve read horror stories.
Oh, how I miss the smell of fried electronics.
I realise that my message needs some additional info.
If you have to clean a connection (for instance, where the jack goes into the socket, such as in the case described in this thread), apply the cleaner to the jack (the male bit at the end of your lead, not the connector in the amp), and then plug it in.
If you have a crackling or otherwise malfunctioning pot (a volume or equalizer control), and you need to clean the contact slider inside the pot, UNPLUG YOUR AMPLIFIER. Don’t just switch it off… UNPLUG IT.
Then, apply the cleaner, operate the control a couple times to allow the cleaner to do its work, and then allow the cleaner ample time to dry – I’d err on the side of caution and give it 30 mins.