Macbook wont backup

Hello,

I have been trying to back up my Macbook Air (13inch Mid 2012) to an external hard drive via Time Machine, however it seems to get stuck on "preparing backup" and wont move on.

I tried it again last night and when I looked this morning, it was still on "preparing backup"

Any suggestions of how I can fix this please?

I am running OS X El Capitan Version 10.11.14 if that helps at all.

Thank you.

Lisa