One thing; don't try to lock the tuners, though you could check to make sure that the fall hasn't loosened the little retaining screws. Strings expand and contract according to temperature and humidity (and age and use), and the machine heads are there so that you can adjust tension to compensate for this. If the guitar falls over, chances are that it will go out of tune, and there is nothing you can do to prevent this (though you could buy a stand for it). It is normal, and happens to everyone from time to time. The day will come, too, when you will have to put a new set of strings on your instrument (they don't last for ever) and practice now will help to prepare you. There are any number of books for beginners which explain all that in detail, with diagrams.

There is an old and rather tired joke often told by performers who find that their instrument has suddenly gone out of tune at an awkward moment. It goes something on the lines of "That's odd; it was in tune when I bought it".