Liese, Ctrl-Z is the magic button to UNDO what you just did. If you use Ctrl-Z five times, it will undo the last 5 commands. A few commands cannot be undone, like closing a file, or saving one, but most of the other commands (merge, change layer, re-arrange layers, add a shadow, move an element, etc.) are all “undoable”.
It is actually the FIRST keyboard shortcut that any PSP user should learn 🙂