2000 Volkswagen Jetta Battery

Engine Mechanical problem
2000 Volkswagen Jetta 6 cyl Four Wheel Drive Manual

I own a Volkswagen Jetta 2000. During the summer it wasn't started for a little over 2 weeks and when I started it, it was dead. I tried to charge it using my other car's battery, it worked, I drove the Jetta for a while, but when I stopped the car, it would not start again. I took my car to a local store so they could check the status of the battery and they told me the battery was fine. I don't know why the charge won't hold. I asked another local business and they told me that maybe the problem is the ALTERNATOR.
It just seems strange that the battery and alternator were working fine before and now they're not. Can it be possible that they simply stopped working because of lack of use?
I don't want to take my car to the shop and get ripped off later.