There is no “factory reset” as such. You’ll have to reinstall the OS, aka “reflash” it. This will wipe everything from your device and make a fresh install.
Instructions, courtesy of @FranklyFlawless : Librem 5 out of memory on Boot partition - #26 by FranklyFlawless
If you’re not running a recent GNU/Linux Debian derivative on your computer, you’ll also need to make a bootable/live USB of one of those, boot into it on your computer, then complete the process of reflashing. (It won’t require actually installing anything on your computer, if you’re using the live USB.)
If you are running a Linux distro already, see if the package uuu is available in the repository, with sudo apt install uuu. If it works, then you can proceed. If not, then the live USB is probably the way to go, even if you’re already running a Linux distro.
Presumably, you tried rebooting and that didn’t correct it?
And the battery had adequate charge?