Malware is an issue on Android phones, and one of the nastiest variants is "ransomware": software that locks up your phone and demands a payment before it can be restored. If that happens to you, there are ways to rescue the phone.
Picture: Daniel Kulinski
Naked Security has a handy overview of tactics you can use to remove ransomware. Generally this involves booting into "safe mode" -- the annoying aspect is that how you do that varies from device to device. Note also that this won't help if you've rooted the device. Hit the post for a much more extensive discussion.
First Aid for Android: How to unlock your ransomed phone [Naked Security]