The quickest and safest method is to reinstall the front end servers again.
However this reinstallation will be done slighly differently, as since your Frontend Server is already registered to the backend, you want to avoid having to re-attach all the applications/consoles to the Frontend server.
For example, if you go to Authentication>Agents on your Administration console you will see references to the frontend services with the attatched applications...
e.g.
In this example, the Frontend Server is called DAS8-2. Please look at the entries on your own Admin Console, and it is important to note down the names of these agents.