koz

Go to Terminal and try to re-install Cyida. If you don't have Terminal, go to search one and install it into your iPhone.

In the Terminal, type the following commands:
su
alpine (this is password for your iPhone terminal)
apt-get remove cydia (you maybe asked to confirm it, just confirm it, it's fine)
apt-get install cydia

and then re-boot.  Hopefully it will work. Good Luck!