I had this happen to me a couple of times with my M 240. Black screen + lock up. I think it has happened twice since I got my M 240 six months ago. Re-insertion of the battery did the trick.
It wasn't a critical failure, and only a temporary annoyance.
FWIW I'm using a Lexar 1000x Class 10 SD card, formatted in-camera. I definitely am a believer when it comes to using fast, high-quality cards. The performance of the M 240 improves drastically with a good card.
The new firmware seems to have made the camera even more stable, I've had no issues whatsoever since the upgrade.