There are some discussions on the forum for this issue and you need to prevent these low voltage situations since next time you may be less lucky and end up with a device stuck in a situation like this
Bug Bounty: Electron not booting after battery discharges completely
Photon can only enter DFU mode with D7 dimly lit, unable to enter other mode
The dimmed D7 LED of death
https://community.particle.io/search?q=dim%20d7
Some threads also suggest some code that tries to prevent such conditions.