This happened to me yesterday a.m. as well. 1st time I've seen it.
I turned on the EPG, saw no listings for the channels & couldn't "select" anything.
Then I noticed the clock was wrong. It was reading about an hour late (showing around 9:35 when it was actually about 10:40).
A reboot fixed everything but it was a weird event I can't explain. I suspect there was a momentary power loss (Edison gives us a lot of them) but not long enough to cause the 501 to reboot on it's own.