Name and email Autocomplete has stopped functioning for me this week

Name and email Autocomplete has stopped functioning for me this week

I've been a happy user for years. I don't remember anyone's email address, I rely on Zoho autocompleting for the people I routinely send email to. This week it's just stopped. Doesn't propose any addresses. 
Is there some setting I have inadvertently changed that I can change back? It's killing me....