You can save your work at anytime by going to menu bar and click on 'save as' a window will open and you can then simply give your work/file a name, make it something easy to remember so you can find it again. However any frames that you have cut will not be saved. As these cut frames are saved to the clipboard so you can undo your last moves. Any frames will be lost from the clipboard when you switch off your PC. So make sure you are happy with the work you have done so far and then you can save it.
Sorry I'm a bit later with the advice, but I've just join the Forum.
P.S. the best thing to do is save your work every hour or day give it a new name, something like new1 and new2 etc. If you save to the same file, you will overwrite your old work. which you might want to keep in case you make any mistakes.