If your jailbroken iOS device is stuck in an endless reboot loop, it is most likely caused by a broken or incompatible jailbreak tweak that you’ve recently installed from Cydia.
In such situations the hard reboot solution also doesn’t work in ending this reboot cycle because the tweak tries to run again when iOS boots up.
To solve this problem, when your iOS device is booting up, simultaneously keep the Volume Up button pressed and you should be able to see your Lock screen again.
Once loaded unlock your device and you can now open Cydia, and uninstall the tweak that you think is creating problems.
In Cydia under installed there is a recent tab so you can view the tweaks installed from most recently so you will be able to select the tweak more easily.