If you have owned and sold at least one iPhone, iPod Touch or a tablet iPad, then you probably knew or were advised to delete everything that your terminal contains/return to factory settings in order not to give the new owner valuable data. You can erase the entire contents of an iPhone, iPod Touch or a tablet iPad by using the option Erase all Content and Settings available in the menu Settings>General>Reset of the Settings application. Activating this function will delete everything that is in the storage medium of your terminal and will returns the terminal to factory settings, deactivating it at the same time.
The procedure is perfect to clean an iDevice before selling it, or to avoid doing a restore in order to get rid of iOS errors or problems, but it is not indicated if you have a jailbroken terminal. Using this procedure in the case of an iDevice that has jailbreak done will put you in the situation of having to restore your terminal, all data from it being deleted in the process. Although normally an iDevice restarts after deleting the entire content, jailbroken terminals will not start, but will be stuck in a boot loop, that is, they will only show an apple on the screen when you try to turn them on.
Unfortunately, the only method to "bring back to life" an iPhone, iPod Touch or an iPad tablet that had a jailbreak before using this procedure is a restore that you can do either from Recovery Mode or from DFU Mode. Before ending, I recommend you to use this option only if you really want to delete everything that is found in your terminal and to avoid it in other situations.
