Remove a Cydia tweak that breaks iOS
If you have version 0.9.3997 or greater of MobileSubstrate…


Turn off your phone.
Turn it back on, then while the Apple logo is shown, hold the Volume Up button.


This enables Safe Mode, disabling all MobileSubstrate tweaks temporarily, allowing you to uninstall the tweak that's causing problems. If you still want the tweak, you can reinstall it, deleting the preference file for the tweak in question to reset it.

To exit Safe Mode, you can respring/reboot your device, or press Exit Safe Mode in the status bar, then press Restart.

If you have a tethered jailbreak, you'll need to hold volume up whilst pressing 'Just Boot' and continue holding even after the Apple logo since Substrate won't load immediately.