If you mount the Librem5 on a PC via jumpdrive you can mount and browse to /boot and there is a .scr file I cant remember the name of, rename it to .scr.bak and rename the older .scr file to not have the date or whatever in the name and it should then be able to boot normally.
I don’t remember all of the finer details but this will hopefully get you close enough. It might be a few weeks before I could give more precise guidance because life…