Check your PC date and time. Reboot if you change it - be sure to check the month/day/year.
If rebooting doesn't bring OneCare back, please don't dive into a reinstall, as you will first need to remove the existing install. And, you won't know what is causing the issue.
If you must reinstall, remove the existing OneCare first via Control Panel.
Then, install from http://onecare.live.com and activate by clicking subscribe and signing in with the LiveID you originally subscribed with.
-steve