On February 11, 2016, researcher Zach Straley released a YouTube video demonstrating how an iPhone can be disabled by setting the date to 1/1/1970. At the time, there was no known way to fix the device. The only option you would have was to replace the iPhone. Apple has since released an update that will allow you to recover your device. You can find those instructions here.
Unfortunately, the news doesn’t stop there. Researchers have discovered the attack can be automated and launched without physical access to the device. By spoofing Apples Network Time Protocol (NTP) to pass the date of 1/1/1970. The performance of the device will progressively degrade until it overheats the battery and permanently rendering the device inoperable.
Please note this should be a stark reminder why you should avoid any untrusted wireless networks. If you have not updated your iPad or iPhone to the latest release, you should do so as soon as possible. Turn off the WiFi on the device until you can update the iOS either over an implicitly trusted wireless network or by connecting directly to your computer and launching the update from within iTunes.