Fran,
This is a very strange problem, but I have seen OE change its Account settings preventing send or receive of email.
I do not know of any way that you can recover your lost data or address book. If restarting does not bring them back, then it does not look good.
The only thing that you can try is to see if the Address book file is still within the OE directory. Go into My Computer or WinExplorer and navigate to the OE folder. Look for a file labeled WAB. If it is there, then you have a chance. ASSUMING that it is present, open OE, File, Import. Again navigate to the file WAB in the OE folder and clck OK.
That should reinstall the Address book.
As for the other email......I really don't know.
It appears that some corruption occurred in your computer, but I do not believe that Avast was the culprit, but since Avast Resident Scanner does monitor the email program, anything is possible.
Good Luck