There are ways you can find out what you changed
Have a look at these logs:
These logs tell you what you have installed with the date and time so it should be relatively easy to find out what you did. Then if you want to downgrade using apt-get you just find out the version you want to install by searching the apt cache for your standard repositories and install that using the version number.
For example if you have package B upgraded you look for available versions of package B in your standard repositories using:
apt-cache showpkg B
which will show you under Provides the version number to use. The last thing to do is use apt-get to install the package with the previous version number, not only the package name (since that will tell you that the latest version is already installed. If you use:
apt-get install B-version-number
referring to the COMPLETE package name you want to install then apt will tell you that the package will be DOWNGRADED. Accepting with yes (y) should downgrade and hopefully fix your problems. First find out what you did searching the logs I pointed too and then take it step by step.