I think what's happened is that your gear cable has become very badly misaligned. You're going to have to either take it to the shop to get adjusted, or figure out how to adjust it yourself.
There are a million articles out there to teach you to adjust a rear derailleur. The basic steps I'd follow would be:
- Shift to the lowest gear (smallest cog)
- Release the anchor bolt shown below
- Adjust the low gear limit stop so that the chain runs quietly and is aligned on the smallest cog
- Tighten the adjusting barrel "most" of the way, so there's still some slack to work with
- Pull the cable reasonably tight (make sure the housing isn't caught anywhere along the frame) and tighten the anchor bolt.
At this point you should have the indicator reading 1 when you are in the first gear... a good start!
Next try changing gears and adjusting the "Adjusting barrel" until you're getting good clean shifts.
Finally change up to the highest gear and then tighten up the high gear limit stop so that you can't accidentally get the derailleur tangled in the spokes by over shifting.