iTunes Add Artwork Has Killed Movie


I decided to add the DVD artwork for movie I have in iTunes (mp4 format film).

I found an appropriate image on Google Images and clicked on Get Info in iTunes, then the Artwork tab and added the image as usual (have done this dozens of times with no problems).

Suddenly, iTunes hung for a moment and now the movie is unplayable.

I deleted it from iTunes and tried to re-add it, but iTunes now doesn't recognise it.

I tried to reformat/convert it in Handbrake, but it says "No valid source found".

It would seem that adding the artwork has fried my video. Is there any way to get it back or will I just have to go back up into the loft and dig out the DVD and convert it again (I'd like to avoid this - I have no idea which box in particular that DVD is in).