Interesting. Not a quick fix then, probably.
The red indicates charging (gets external power) but there is also the possibility that it’s not getting enough power. For a check, you could try letting the phone charge for a while, before trying to boot. And if I remember correctly, L5 stops charging the battery if the heat sensor trips, so the previous charge may not have been that much. Red also indicates that something is alive.
If the phone doesn’t boot with just the cable but the light come on, I’d try to connect to it using the jumpdrive procedure next. Have you tried it before? See: https://forums.puri.sm/t/possible-to-make-save-librem-5-distro-data-as-an-image-file/ That should let you see inside and let you copy any files you may want to save (also, lets you take a back up of the whole disk, in case you do the next bit). If there is some filesystem fault, you could also use it to re-flash your phone. Jumpdrive will also tell, if there is more serious faults (like, say, with the screen, as it would display a message there about jumpdrive).
Battery could maybe be dead even if it seems ok. But let’s assume for now, that it’s good.
Of course, you always have the option of contacting support via email.