I am using Avast Free v2015.10.0.2208 on Windows 8.1. Today I saw by chance when passing by the service bar that Avast service said "You are unprotected". Clicking on "Start now", "Resolve All" didn't help. Shutting down and restarting the computer didn't work. Clicking repair in control panel/uninstall software/Avast didn't help either.
The strange thing was that the Windows security center didn't give an alarm an even pretended that Avast would be turned on. Finally I opened the task manager and found that the service "avast! Antivirus" was indeed stopped. After right clicking the service and choosing "start" the service restarted and Avast turned back to life.