One of my favorite features about Mac OS X is the built in VPN Client that supports L2TP over IPSec, PPTP, and Cisco IPSec. Unfortunately it decides to get buggy once in a while preventing me from creating a VPN connection into work. One way to remedy this situation is to restart the machine, not a very good option when your're trying to get work done.
The other method is to open up the terminal and do some command line mojo. The command below will resolve the issue and get me up and working quickly.
This will re-enable the service. Try reconnecting and all should be good.