jml,
What has happened, it this. Your firewall was asking for permission to connect the program (Avast) to the internet.
Since you said NO, it dropped the attempt, but you made it worse by telling it to remember you answer. Had you not done this, the next time the program attempted to access, the "popup" would appear and you could have said yes.
Now you will need to access the firewall's Applications or Programs menu, and find out how to "release" Avast. Usually highlighting the program you want to change, and right clicking on it should show the options.
If you tell us here which firewall you have, one of us should be familiar with it and can give you step by step instructions.
Techie