Im getting avast trojan blocks popping up when i try opening facebook?

i dont know how this started, I was on facebook and then in the corner, an avast trojan block pops up and doesnt let me on facebook, webpage saying 'server not found', and whenever i try to open facebook, it does the same thing? its happenin on youtube aswell?please help me out. ive noticed other people are experiencing same difficulties,anyone know how to solve this?thanks/