Hey Lsemmens
plug the iPad into a computer with iTunes open, hold both the power button and home button down and count to 10 - at this point release the power button and wait for the windows beep - at this stage you should be in DFU mode and iTunes should ask to restore/upgrade
be aware this will wipe the device and if you get an error - post it up, I have some experience with a few rotten apples
f
Bookmarks