There is no firmware update for PRS300, I checked on their website www.esupport.sony.com. Maybe sometinhg got corrupted on the internal memory. You can try to do a "Hard Reset" on the eReader. This will set the unit to factory settings and will delete all content:
1. Disconnect the Reader
2. Hit the Reset button at the bottom of the Reader
While the reader shows 'Opening Book...' press and hold the number '0'
(Zero) and the 'Down Arrow' both at the same time until reset all
4. Once you do that you will get a confirmation saying that all content will be removed
you just press number 5 to confirm
If this does not work, contact Sony for service options =(