I have Norton AntiVirus/ Says my computer is at risk/SONAR Protection?

Does anyone know what this is and how i can get it to go away, i am not sure what it is, i did not do anything different then normal, but when i shut my computer down and came home and turned it back on it would not turn on said system reboot, and then it was on. But now it say i have this SONAR Protection virus anyone know what i could do. Thank you. Oh and i am not the best with computers so i need a lot of detail on what i am suppose to do, thank you.