Hmm first thing to do is search these forums for you answer. There is a great search function.
Second, if you can't find your answer, give us more details. What IOS are you running. Are you using a Mac? Etc.. Etc..Screen shots, anything that might help.
if it's the (-20) error, this happened when I went from the 3G to the 4. I had created my 4 with the 3G backup then my 4 gave me the error. I was told to restore my phone and when I did this, I lost everything. So what I did next was to go ahead and delete my old back up file which I couldn't use anyways and the next time I synced my phone, it created a new backup and all is well.
