Anaxilus is right. "Temporary bricking" is an unfortunate occurrence with rockboxed clips. It happens to mine on occasion. I try to turn it on and nothing happens.
To fix it, I typically let go of any buttons for a few seconds. Then I press and hold the power button for a full 30 seconds. Then let go for a few seconds, and press it again. It turns right back on.
Annoying, but it doesn't happen often, and rockbox is still worth having that occasional annoyance.