Take a look in "Open/Share Solutions" (ctrl+o on Windows). Check "Show auto and quick saves" and see if your high score is there.

Otherwise, from the Undo menu, "Restore credit best" (or the lowercase "I" hotkey) may find your high score.

Sometimes, solutions seem to get lost. I've seen several cases where the client shows a high score that can't be found through any of these methods. In a few cases, it was possible to get back to high score with a little work, but in other cases, the high seemed to be a mirage, impossible to reach.

All solutions, including creditbest, auto saves, quick saves, and manual saves, are written out to .ir_solution files. The file and directory structure wiki page explains where to find these files. Unfortunately, they're mostly binary, but you can see things like the title and description of a manual save. There's no way I know of to get Foldit to load one of these solutions except using the menus and hotkeysu.

See also this blog post.