Update to Windows 10 causes Avast to cease working

A member of my family used to have Windows 7 on her computer, but last night it automatically updated to Windows 10. Since that update, she can’t access the Internet, & gets an error msg to the effect of:

Error #10013
Avast Service (AvastSvc.exe)
Blocked by personal firewall

What could be the problem?

Be sure that AvastSvc.exe is allowed in your FW.