Most likely the update has overwritten some config files. First you should check if mysql is working, e.g. with this command:
netstat -tap | grep mysql
If mysql works, then you can use the ispconfig update script to rebuild the config files. Download ispconfig 188.8.131.52 tar.gz, unpack it and run the update.php script which is in the install folder. Choose "yes" when the updater asks you if the services shall be reconfigured.