this whole thing is a nightmare, at least in windows. I had my phone connected and everything was ok. I have a Samsung Galaxy, I\'ve installed the USB driver and it was okay
I ve seen multiple times this problem and there is no common answer. i'll make a list of solutions that I have seen across severals forums:
Restart ADB server with the following commands: adb kill-server and then adb start-server
Change the USB cable you are using: I've seen a lot of complaints about Samsung's USB cable.
Change the USB socket you are using
Try to reinstall the drivers (for instance, for Samsung devices, installing Kies; this solved the problem for me)
Try to reboot your phone
Unset the debug mode on the device and set it again
Adb seems to often cause troubles, not the best piece of software I've seen!