Erased videos & impossible to restore back up

On syncing my iPad to my iMac 5K some home movies were immediately erased from the video section of my iPad. I tried to restore the latest back up but the computer says "insufficient space on the iPad - which I had just backed up ! Are my videos lost? What do I do to restore ? It is a 16G iPad 2 which was nearly full - hence the attempt to get the videos off the iPad & onto iTunes. I fear I have failed on all points !!