Here are instructions on how to reset the PRS-300. *note* this should erase all contenet loaded on the device, but should also restore it to working condidtion:
https://us.en.kb.sony.com/app/answers/detail/a_id/31628
In future, in case you were unaware, pleaes make sure to eject usb devices/drives before disconnecting them. This especially includes any Reader device Calibre is managing.