Today I upgraded to Ubuntu 15.10, no errors happened during the upgrade but after the upgrade I can log in using my username and password but unity does not work. No launcher, no panel absolutely nothing just the background.

I tried to start unity myself using compiz --replace unity it gives me an error about a missing function definition in libunity.so and it fails to load.

Please can you help me with this?

[UPDATE]

output of sudo apt-cache policy libunity9 is

libunity9:

  Installed: 7.1.4+15.10.20151002-0ubuntu1

  Candidate: 7.1.4+15.10.20151002-0ubuntu1

  Version table:

 *** 7.1.4+15.10.20151002-0ubuntu1 0

        500 http://sy.archive.ubuntu.com/ubuntu/ wily/main amd64 Packages

        100 /var/lib/dpkg/status

[UPDATE]

After trying to follow the two answers the error message has changed as follows

DISPLAY=:0 compiz --replace unity outputs to standard error

  compiz (core) - Info: Loading plugin: core

  compiz (core) - Info: Starting plugin: core

  Invalid MIT-MAGIC-COOKIE-1 keycompiz (core) - Fatal: Couldn't open display :0

  compiz (core) - Info: Stopping plugin: core

  compiz (core) - Info: Unloading plugin: core