My Time Machine Back up is stuck on "Preparing Back Up". How do I fix this?

I have done the following to try and fix and nothing seems to work:

1. Stopped the back up
2. Removed the filed
3. Emptied the trash
4. Rebooted the computer with the external hard drive attached
5. Retried the back up again and still stuck on "Preparing Back up".

This has been going on for over 13 hours. My previous back up on 6/23 worked perfectly fine. I do a weekly back up but have not been able to do one since. Any suggestions?