Look first tried to upgrade via SEUS. Maybe he will fix your problem. If not, then you have 2 options:
1. If you have your phone under warranty , take it to service center.
2. If not, the make a full flash ( main, filesystem and custom file )
You can find all files in download section.
Here you have :
Step by step instructions for flashing a phone using only XS++
Flash your phone using SETool 2 Lite
Custom files - description
Languages
Nice day