We are currently switching the depots for the game, rendering the game largely unplayable because of an unforeseen issue.
We are currently working on solving it. This is on our end, and you do not need to reinstall the game, verify game cache or any of the sort. Steam will automatically update once we fix the issue. This is not the 3.0 release, but it is preparations for it.
Sorry for the inconvenience,
- Zombie Panic! Development Team