[[sammich]]

You'll be surprised how much lint builds up in the charging port if you don't have a case. I second darren's suggestion.

Also, wiggling around the cable may help if there's a contact issue.

If nothing works, I would try hard-resetting. There are certain settings within the phone that allow you to specify how the device charges (such as disabling data transfer when connected to a computer and only charging), so it is possible that something was changed unintentionally.