Hope you backed up your data... that's most likely a virus. Usually those viruses encrypt your phone, and demand ransom (sound familiar?). Get your phone reflashed, and you should be fine. Currently not aware of anything available to the public to combat ransomware, but that's something I'll probably start looking at next weekend... would be nice to be able to fix phones on my own, if ever needed.
When your phone gets fixed, consider using Cheetah Mobile's SimpleLocker Cleaner (I know it's available for Android, not sure about iOS). Cheetah also has a decent reputation for their mobile protection, so consider using their AV once things are cleaned up. For the price of free, it's definitely better than using no protection. If you want to pay for mobile security, then there are a few other options to consider.