given the time-stamp of this thread, I guess it's solved. But the same thing happened to me today in the morning - a black screen, "Reader" logo, barely distinguishable "loading e-book" (or something of that sort) under the logo, and absolutely no reaction to on/off switch or reset button.
Charging the reader for about 2 or 3 hours (via USB cable) did the trick - it became alive again on its own. I turned it off in the menu then and charged it till the red light switched off. The only "problem" was that the date and time were reset.
Works fine, now.
I hope this note helps other desperate PRS-300 users who find their device frozen as I did in the morning, so that they don't panic as I did