In my subfolders, under All Mail, I had a large file, with many subfolders, full of vital information, thast has disappeared.

I was working busily in Thunderbird email, where I have a number of subfolders under "All Mail" where I store replies and such. One of the most important subfolders (which itself has many subfolders) has suddenly disappeared. I've looked manually through other subfolders, to see if I somehow moved it by accident, to no avail. Is there a way to search for a subfolder, or can anyone suggest where it might have gone?