"Unfortunately, System Ui has stopped" when i open my phone

[Android Central Question]

I have a HTC desire 820 ! When i try to open my phone this message pops up :
Unfortunately, System Ui has stopped ! I can't access the home page or anything ! When i click "Ok" it just closes then opens up again ! Not reboot just a normal closing.
If anyone could help me somehow i would highly appreciate it ! And i repeat ! I can't access any of my apps ! Thanks !