04-19-2019 12:00 PM - edited 04-19-2019 12:01 PM
First off, formatting the player will not erase the firmware, but it sounds like you might have Rockbox installed; do you? I believe formatting WILL delete Rockbox, except for the dual bootloader which is actually modified into the Sansa firmware when you install Rockbox.
This dual bootloader allows you to boot into either rockbox or Sansa (original) firmware via button presses. It will boot into rockbox by default. If this is what happened, your player is trying to boot into rockbox, but can't find the file (since it's be deleted). You will have to to boot into the Sansa 'side' (by holding the rewind button, if I recall correctly) during start-up, then plug to your computer . Then you will be able to copy the .bin file to the root directory & update (reapply) the firmware (minus the dual bootloader).
Once this is done, your player will be basically back to factory status. If you want Rockbox again, you will have to re-install it.