Problem with my guitar?

It would have helped a lot if you'd told us something about what type of guitar you have.

Assuming that it's not something REALLY strange, the tone will be controlled by a knob that you turn (it might have two tone controls). The switch you mention (with three positions) will be a pickup selector switch which, I presume, allows you to select neck pickup, both pickups together or bridge pickup.

If you have separate volume controls for the two pickups, selecting a pickup with its volume control turned down or off altogether will mean you get very little volume or none at all.

Otherwise there is a problem with the electronics - most likely the switch but there are other possibilities.

Without more informaton or a chance to try the guitar it's impossible to be more helpful.

You most likely just have a bad switch. This is an inexpensive repair. You could have an issue with the one pickup or it's wiring, but most likely just a cheap switch. If any shop is telling you to buy a new guitar rather than spend $20 repairing yours, then you need to find a new shop. If you are pretty good at soldering you can replace this yourself.

Tony B has good suggestions. Is this a les paul type with four knobs and one switch? I suggest using make and model number on google search so you can learn what knobs do.... I thought bad switch too, but hopefully just your lack of knowledge regarding tone/volume controls could be culprit. How old is it? Do you or friends have even slight ability to solder etc? could be loose contact (close observation, some contact cleaner or emery paper can get corrosion off) or bad joint. Amen to finding new shop over such a simple to fix problem...