When you disconnected the 505, did you just unplug it or disconnect it using the icon on the reader software first? Not sure why downloading the software to the reader would've caused this problem, but if you have a Windows computer, can you get to the 505 drive and delete it off via Windows explorer?
Have you tried turning it on without the SD card?
I never tried doing this, but on a Windows computer, can you go into the reader through windows explorer and delete out all the books on it to see if that helps? (Might want to see if someone confirms if this is possible and is something you can try since I never done this before)
Maybe someone can give you some instructions, but it seems like you might want to try formatting the 505 since it might be a corrupted file. Means lost of data on the reader though.
Again, please wait to see if people recommend what I've mentioned as I've never tried doing any of the above through windows explorer and not sure if it would do more harm than good.
I hope you find a solution!