Hey Dog! You may have lost your aircraft due to two different reasons:
1. Your rank is too high to fly them unless you pay premium and join the WW1 server. You'll see "Old" instead of the regular arrow button (like what happened to all WW1 aircraft in my case).
2. Zuperman just made both aircraft (the S.E.5a and the Dr.I) playable only by premium users, exactly what happened to the Fw 190: it was a free plane until this was edited less half a year ago (shortly after I finished upgrading it... still hurts in my wallet). You'll see "Premium" instead of the regular arrow button.
I've checked your stats and you're rank 15, so I suppose it's the second one.
About the "loading" sign, I suppose you mean what happens when you press the "play again" button: the game sometimes seems to be frozen in this screen.
However, the game isn't frozen. You can still press the play button in the lower right corner to enter in a game "normally".