If the damage is very light you can fix it by retapping the same size. Chance of success - about 10%. Proper engineering solution - fit a Helicoil insert of the right size. If it can be done, chances of success 100%, cost about £10-20. If it can't, drill and tap next size up, cost £10-20ish, if the insert has enough meat this will be 100%, if not it won't work. The bike shop may know the chances of this succeeding. How do you tell? You have a go...
Finally, there is a nasty bodge. Purists, look away now. This is only recommended for old bikes of low value. It's a bodge. Find a screw, any head, and quite long, that will fit in the hole and sort-of tighten up. Mix up a very small blob of Araldite or similar and put it on the threads. Wind it in with plenty hanging out and leave it to set. When it's set. cut the head off, and mount your bottle cage with a nyloc nut of the right size. Cut off the excess, file the end smooth. This screw will never come out so once it's in, it's there for ever. You can undo the nut but there will always be a threaded end poking out. Sorry purists, I know it's nasty, but it works.
SAFETY: DO NOT leave stray bits of threaded rod hanging out of your bike, once the component is on either file them smooth or fit a Nyloc nut which has a rounded end. In a crash exposed screw ends can make a hole in you, one of my friends needed surgery when he put a titanium seat clamp screw through his knee.
