Have you recently changed USB ports (if its hooked up that way.)?
Here is a blurb from Andy's site:
Problem: (USB Only) Board appears not to work until WinIPAC is run every time. Yellow exclamation marks in Control Panel, Device Manager.
This is a rare problem, and is caused by a registry error. It is suspected that this problem occurs if the board is regularly swapped from one USB port to another, or different board versions used on the same PC. Microsoft have been alerted to this problem. To fix this problem, download this file: Ipacfix and click on it. This deletes all references to the I-PAC in the registry. Then unplug and reconnect the board. If you would rather do this manually here is what to do:
The fix involves registry editing, the usual caution should be observed.
Run regedit, and open Hkey_Local_Machine/System/CurrentControlSet/Enum/HID and delete the keys containing VID D209 Also Hkey_Local_Machine/System/CurrentControlSet/Enum/USB and delete the same keys. You might have to right-click and set permissions to do this. Unplug and re-connect the board.