My vote is for shot down or your receiver lost power. It doesn't necessarily mean the battery is at fault because usually it is the switch. Let us know if you ever find the airplane and then we can help you go through checking out each part of the radio system. I had a similar incident with my pride and joy extra 300 a few years back. I didn't secure the battery and it pulled itself out on an outside loop. She went straight in... Ouch...

Failsafe may have helped some, but it doesn't work wonders. If someone shot you down, and you had failsafe, you could program the failsafe to hold the last input or tell all the servos to go to neutral. At this time you could start screaming at the top of your lungs and hopefully someone realizes their mistake and turns off the radio and you can start flying again. As far as keeping power to the receiver, a redundant switch system, battery, digital failsafe switch (that fails on instead of off) could all help. I hope this helps out. Hang in there, crashing happens, but the hobby is totally worth it.