Sometimes when jailbreaking an iPhone or after installing a jailbreak app, your iPhone will get stuck on reboot, or will hang on the Apple logo. This problem happened to me before and it was kind of a nightmare. In order to get your iPhone back on track, you will have to put it in DFU mode (Device Firmware Update mode). This will help you restore your iPhone without loading the OS first.
Putting you iPhone into DFU mode should be used as a last resort if your iPhone is either stuck on the Apple logo or keeps rebooting.
1. Connect your iPhone to iTunes.
2. Hold the Home button and the Power button at the same time for 10 seconds.
3. After exactly 10 seconds, release the Power button but keep holding the Home button for 10 seconds or until iTunes pops up a message saying that it has detected an iPhone in recovery mode. At this point, click OK, then click “Restore” in iTunes.
Note that it often takes several attempts to get your iPhone in DFU mode.
Check out this video for a detailed explanation.
Thanks @CeciMartin for the inspiration.