The below is information I've picked up from other troubleshooting threads...
First of all, I know that charging with a mains (AC) charger for a FULL charge is the first step.
Then, if you have an ebook, pic, or music, on your Reader, make sure you have a copy on your computer so you can reload it again.
Last make sure that you don't have the music player on (even if paused it still uses battery).
I've been lucky and have never had to do a soft nor a hard reset. Additional information can be found in the MR Wiki below..
http://wiki.mobileread.com/wiki/PRS505
Trouble shooting section...
https://wiki.mobileread.com/wiki/PRS505#Trouble_Shooting:
And the see also section for the manual link..
https://wiki.mobileread.com/wiki/PRS505#See_Also