Could it be a battery issue? Defective battery??
Any thoughts would be appreciated.
Since you replaced the props and it flew the next day or so I vote for the battery becoming disconnected especially if you were doing any quick lateral movements.
One of my 3 batteries won't click in all the way on the top click but it hasn't fallen out yet. I was thinking about putting a putting a piece of white electrical tape I have across the back of it. It sticks well to the Phantom plastic. Another of the batteries has to be "rammed" in to close the gap all the way. In any case, I always give a tug on them to make sure they'll stay connected.
Like others have said it's probably a good idea to hook it up to the software and check all your calibrations after a hard landing (AKA as a crash).