From the PRSCustomizer site:
http://www.prscustomizer.com/prscustomizer.html
Recovering from errors
In the PRS 505, Sony has produced a very robust device. It's actually
very hard to cause the device to become unresponsive, otherwise
known as a "brick." However, when making complex changes to your
Reader inadvertent errors on your part may cause it to become
stuck on the "Reader starting up..." screen. Recovery is usually
simple.
Recovering a brick
1. You're stuck at "Reader starting up..."
2. Use a a paperclip to press the reset button on the back of the Reader
3. Quickly slide the power toggle.
4. When the Reader reboots, immmediately connect it to your PC via usb cable.
After a minute or so, the usb connection message should appear; now
you can use the US/Canadian Sony Updater to reflash your Reader.
When finished, your Reader will be using Sony firmware 1.1