programmer is not responding

Hello, i'm new to arduino,
I've got my new arduino last week and worked fine with my mac, now after a week it stopped reciving sketches from my mac and returned avrdude: stk500_recv(): programmer is not responding.
The weird thing is when i'm trying to upload sketch from my other Windows 7 machine - then it works fine.

I wondered what is the problem, and if it can be fixed? How it can be fixed?
Tnx,
Noam Cohen