I have a Rockboxed Clip (not +) that just spontaneously slipped into a coma a few weeks ago. I tried holding the power switch for 30 seconds, a minute, five minutes - nothing. Nothing showed up on the computer (tried on a Mac and a linux PC) when I plugged it in. Just totally dead.
I used a rubber band to hold the power switch overnight, and that fixed it. When it froze it had maybe 80% charge, and after a night with the power switch held, it was almost completely discharged. It's still working OK.