Strings need a while to settle on the guitar. On guitars with relatively low action, that means they buzz. If your guitar spent too much time (depends on the guitar) with no strings, the neck might have relaxed and it is being pulled to the ussual tension. However, your neck may be distorted, frets worn or out of place, truss rod too loose or too tight. :eek:
If you have no experience with these, have your guitar checked by someone else, who can explain what's the problem and how to solve it should it occur again.