This is not only happened with you thousands of people are becoming victims. So, you no need to worry about this you can recover it back very easily.
Step 1: First, factory reset the phone.
Step 2: Still you face the same problem boost your mobile.
Step 3: Or you can go and upgrade your software in the service center.
Note: Install some best antivirus (Kaspersky, McAfee, Comodo) so that you can avoid this kind of attacks in future.