What antivirus do U have?
Do U still see the antivirus icon in the taskbar? If so, right click it, or double click it and check for the option to enable it.
Or it may have been stoped from services. Go to Start>Run and type msconfig and press enter
Go to services tab and see if U can find your antivirus there and check the box next to it. Also go to Startup tab and do the same.
Or check if U can find it in Program Files, or it might be uninstalled. Check Control panel >add remove to check if it's listed.