Backup quit working

I have a windows 7 64bit laptop and I have Norton 360  version 6.4.0.9. Its time do a back up.  It wont complete it gives me this error   "There is a backup problem.  Make sure the destination drive or device is connected then try again".  I have tried everything and it always gives the same error.  I am using an external hard drive and it shows that there is plenty of room yet.  what do i do.

miss frustrated:smileyfrustrated: