Sounds like a lot of essential packages have been removed. First you should reinstall the meta package for the server’s type which should pull in a lot of the other essential packages.
First determine your server type: ucr get server/role If that doesn’t work (e.g. because ucr isn’t found anymore) a simple grep server/role /etc/univention/base.conf should work, too.
Now install the corresponding univention-server-… package, e.g. univention-server-slave for the role domaincontroller_slave. Take a look at apt-cache search univention-server- if you’re unsure.