Hi,
don't panic! Take another deep breath, PRS-505 is (nearly) unbrickable.
First download the Sony FW-Updater (eg. from
here)
Reset your reader, restart it and immediately connect to USB. (It dosn't matter that the volumes are not readable.)
Fire up the Installer (assuming a Windows OS) and try to update the firmware.
If this fails, there is the 10 times reset methode, to set the reader in firmware update mode.
Reset the reader, flipp the powerswitch and on the first flashing of the screen reset again.
After the tenth reset/restart a message about necessary firmware update should be shown.
Connect to USB and use the Firmware-Updater mentioned above.
HTM
Mark