"In normal circumstances there is NEVER a need to run rpi-update as it always gets you to the leading edge firmware and kernel and because that may be a testing version it could leave your RPi unbootable". https://www.raspberrypi.org/forums/viewtopic.php?p=916911#p916911
Even the rpi-update documentation now warns "Even on Raspbian you should only use this with a good reason. This gets you the latest bleeding edge kernel/firmware."
sudo apt-get update; sudo apt-get install --reinstall raspberrypi-bootloader raspberrypi-kernel
will put it back to the latest supported kernel/bootcode.
You COULD try rebooting to a root shell and can repair most such problems on the Pi.
- Append
init=/bin/sh
at the end of cmdline.txt
and reboot.
- After booting you will be at the prompt in a root shell.
- Your root file system is mounted as readonly now, so remount it as
read/write
mount -n -o remount,rw /
You can then edit files or restore kernel/bootcode.