Firstly I would like to thank you for your responses.
The solution was given by DavidR, suggesting I use the Avast Repair selection, which I did.
But the first time it successfully repaired it, but nothing happened, the Web Shield was still showing OFF no matter what I did.
After fiddling around, I carried out a successful repair once more - still no change. Then poking around the settings and menu's etc. I saw a red notice saying my PC needed rebooting, which I did. When the PC finished rebooting, I checked the Avast status page and the Web Shield was ON !!!. All back working normal again.
So problem is now SOLVED.
This whole trouble was caused by the latest update to version 18.6.2349, build 18.6.3983. Up to that point my Avast had been working OK for years with no trouble.
THANK YOU EVERYBODY.