If you have any add-ons that modify the main menu, remove them or unsubscribe from the workshop item.
Should that fail, unsubscribe from all workshop content and delete the add-ons folder located in "(steamapps)/common/Left 4 Dead 2/left4dead2".
As a last resort: Close Steam and delete appmanifest_550.acf from the steamapps folder. Start steam back up and select to Install Left 4 Dead 2. It will detect all your existing files and not download anything unnecessary.
You should have discussed this matter with the group leader or a higher-up when you caught the cold. I know this doesn't fix the matter, but in the future, it's always better to let people know ahead of time when you catch something.