Half of my iMovie videos disappeared, can I retrieve them? I have filmed myself doing a painting, it is a lot of film (20 hours). I was planning to massively cut it and speed it up to make a sort of time lapse. 

The problem is the first half of the project has disappeared. It was all filmed in the same project but the first half has just gone. I'm quite gutted as I really wanted to do a film of me painting to put on my website. The thumbnails are still on my machine but I can't find the actual clips. Has my mac dumped them or are they hidden somewhere? Can I retrieve them?
I'm not sure how you filmed yourself doing the painting, maybe with an iPhone?  If so that's where I'd look.  Maybe you mean a digital painting and you were doing a screencast?  It's important to describe which hardware/software you used as it will determine the location of the files.  iMovie, I believe, you could have only used for editing.

As an aside, I can recommend using the Procreate iPad app for recording artwork, the feature is built in, you don't have to do anything to record as it happens automatically, and the resulting screencasts are very high quality and with good timing.