To resolve your sign in issue, please "over-install" RealPlayer. Over-install simply means installing RealPlayer over the existing RealPlayer. When you go through the installation process, you will be prompted that RealPlayer is already installed, simply proceed with the installation when prompted.
With respect to your library, as long as the files are stored in your PC's hard disk, you should be able to add it back to RealPlayer's library. Here are some instructions to add files back to the library.