Here are the instructions to turn it back on:
Click Start > Control Panel.
Double-click the Security Center.
In the right pane, click Windows Firewall. The Windows Firewall appears.
Click OK to close the Windows Firewall.
In the left pane of the Security Center, select Change the way Security Center alerts me.
Click Alert Settings.
Select Alert Settings, Firewall, and Virus Protection.
Click Automatic Updates.
Exit the Security Center.
Let me know if that doesn't work...