Guitar input jacks--where you plug the cable into your guitar--can become damaged or be pulled free from the body of the guitar. A couple of things can cause a guitar input to go bad. The first is that one of the two wires connecting the web of wires between the pickups and volume pots to the input has become frayed or has simply snapped. The second is that the jack, which has a receiver for each section of the TRS input, has snapped or is bent. This can mean an expensive trip to a musical instrument repair shop, but with a few tools and a steady hand, you can fix a broken guitar input.
Locate the input's face plate; look for what you can unscrew that's closest to where you plug the instrument cable into your guitar. On a Fender Stratocaster or similar models, you most likely will have to take the pickguard off first; unscrew all the screws and carefully slide it off. On a Les Paul, or any model designed to look like one, the input face plate is usually on the back of the guitar.
Unscrew all the screws necessary to remove the piece. Be careful not to strip the screws; they are often difficult to replace. Store the screws in a plastic bag so you won't lose them.
Slide the pickguard, or rear faceplate, off the body of the instrument. You may have to stick a butter knife or palate knife under the edge and give it a little pop. Set the piece aside.
Remove your guitar's input jack from its slot that runs through the body of the guitar. Try to plug a TRS (1/4-inch) cable into it. The TRS plug should go through the outer ring and sit snugly against the contact for the tip of the plug. If it goes in properly, you will feel it click into place, and it will hold there under its own power.
Replace the TRS plug receiver if the TRS cable does not go in properly, meaning your TRS plug receiver is bent or broken. Cut, strip and remove the old wires, making sure to note which wire goes to each section of the TRS plug receiver, and solder the device back together.
Check the two wires--usually one red and one black--that connect to the input jack where the connection takes place. It is possible that one or both of them have become frayed, partially disconnected or have come off.
Desolder, clip the wire to leave about 3/4 of an inch of open copper wire exposed, wind it around its contact on the TRS jack, and solder it back on.
If you can't find anything wrong with the input, the problem is likely in your pickups or somewhere else in the wiring within your guitar. If you do not have significant understanding of wiring systems, take your guitar in for professional help. Any large musical instrument retailer, and most smaller music shops, carry TRS plug receivers tailor-made for guitar application. When you go shopping, bring your old one with you for comparison purposes. The part typically sells for less than $5.
Use lead-free solder to avoid health hazards. A high silver content is best, but you do not need expensive solder for this application.