Exactly the same thing happened to me – twice.
The first time, I’d reached level twelve, and just got the second key.
Along came a video, and, crash, kicked me clean out of the game.
(Yes, also on a Kindle Fire HD)
I logged back in, and the game was back to the introductory video.
I checked that I was still logged into my Rovio account (and Facebook) – I was.
I logged out of both, restarted Kindle, restarted game – still right back at the start.
I logged back into Rovio and Facebook – STILL right back at the start.
Support were rather quiet, so, out of boredom, I started again.
This time, I got to level 6, and along came a video… you guessed it. Crash!
Now, I’m back to square one again, and I’m sitting here wondering how long it will take support to respond, and give me back my progress AND my purchases.
So…
I WAS logged into a Rovio account. It looks like when it reset, it updated my account to say that I was at the beginning of the game. I’ve also tried logging into the game on Bluestacks since them – it’s identical to my Kindle – right back at the start. The really strange thing is, when I logged into Bluestacks, it updated my Google Play achievements with everything I’d done – right up to the second key! Weird. And annoying.