Once you have an iDevice in re overly mode, there is no way to get it out unless you connect your iDevice to a PC or MAC with ITunes installed. Connect your iDevice VIA USB.
If you don't already have ITunes installed, you do so by going to www.apple.com/iTunes and install it from there. The installation shouldn't take over 10 minutes.
Once you have hooked up your iDevice, open ITunes on your computer. Then in the menu you should see a symbol of your iDevice, don't worry about this step if your iDevice is showing on the screen.
Once you have got your iDevice open on iTunes, it should tell you that it's in recovery mode. Press on Restore.
Unfortunately, if this is the first time connecting you iDevice to your computer and/or you haven't saved a backup of your iDevice to either ICloud, ITunes or you computer, you will have to restore to factory setting.
If you have backed up your iDevice, you can press Restore and Backup. From there, depending on the speed of your computer, the process of restoring your iDevice may take up to 8 hours. Fortunately, if you have a fast computer, it may only take 1/2 an hour.
Thank you and good luck!