Time Machine doesn't work!

I have a 500GB external HD that I use for Time Machine. For the past two days now it has said "Preparing Backup..." in the top right hand corner. I have unplugged it numerous times and it still says this. Also the last back up date was January 27, 2014. So it isn't that I only look at it on the hour when it is doing it's auto backup.

Please help me!