Another possibility which may or may not be relevant to you.

Usually when I am scripting something, I do it in a completely
different mirc installation than the one I use for chatting.. On one
pc, I have around 6 installations of mirc on the c drive, and several
on my d drive - each left over from different projects I had started.

To top this off, I have another pc and a laptop which similarly will
have the same, and I frequently switch between these depending
on what room I feel like working in, or whether I feel like sitting on
the bed and scripting something.

It's not uncommon for me to (temporarily) lose something I've
worked on,.. simply because I'm not sure where exactly I was
working on a snippet of something. In addition if I was working on
the same project on all 3 computers, only one will contain the latest
version - the other two would be less complete (or if I had
forgotten, would seem as if code had dissapeared).

Could this possibly be your situation?, or are you a one-mirc-only person.