Yes, a mains charge, preferably overnight, should sort this out.
The battery got flat because the reader uses a small amount of power to maintain settings etc. Even unused, it will run down in about 3 weeks.
However, you can do a complete shutdown with the 505, in which case it won't be using power, but will take longer to reboot when you do start it up.
(If an overnight mains charge doesn't sort out the problem -- and it usually works -- the next step is a soft reset.)