If you formatted the player, the .rockbox folder with all the operating files, etc. was erased. However, the SanDisk firmware (which does not get erased during formatting) still contains the Rockbox-modified bootloader. This means it will try to start up Rockbox but can’t because the files are now gone.
You need to boot into the OF (original firmware) by holding the REW button while powering on and then either manually install a fresh (non-altered) version of the firmware if you want to continue on with only the original SanDisk firmware, or re-install Rockbox.
Either way, it’s an easy fix… 
