Cisco IPsec VPN Negotiation Fails After Network Disconnect
The problem is that racoon, Apple's proprietary Cisco VPN client doesn't close itself gracefully on disconnect, this actually makes all Cisco VPNs stop working if one was connected when the network failure occurred.

To fix this, simply stop racoon:


Open Terminal (/Applications/Utilities/Terminal)
Type sudo killall racoon (you will be asked for your password)


You may now quit Terminal and reconnect the VPN as normal.