Unplug the power and let the battery drain to force shutdown - it takes around 2 hours for a fully charged battery to drop below 50% for it to shut down. Once shut down, plug the power back in and let it charge for about 10-20mins for the battery to reach over 50% then boot the machine back up. It will then initiate the BIOS recovery. If you can't wait for 2 hours, you'll have to unscrew the back plate to disconnect the battery.
I restarted my dell latitude 3420 to update my firmware and it is stuck at: flash progress 0% saving manifest. I don't know what to do. It has been stuck for a while now.
Latitude 3420, stuck on firmware update