Don't know if this has been mentioned before, but it happened to me again today, so thought I would mention it. Sorry, but don't have time to look through 40+ pages to check.
This freaked me out when it first happened, as I thought my 550 had died. After powering down, could not get it to turn back on, no matter which buttons I pushed. On a whim, I removed the battery pack and reconnected it. This reset the unit, and it powered back up. This has happened about a dozen times in the past 14 months, and for no particular reason, and the battery pull always has fixed it. A couple of times, the reboot button sequence took care of it (pushing the top left and bottom right buttons at the same time), but usually no.
So, I always make sure to carry the allen wrench that fits the battery pack screw.
Ed