Apple mail can't send messages. "wants to use private key in keychain"

Hi

for a few months I have been getting this message every time I try and send a message on apple mail. I looked it up a few months ago and the advice was to go into keychain and delete private keys. This used to work for a week or so before it came back. Now keychain seems to have complete disappeared and there is no such thing in the passwords app and the advice I could find is that such things have moved into the registry which is not something I am competent to look at. I am just a basic email user - I have never intentionally set up any keys. Apple mail is now completely unusable. If anyone knows how to solve it would be greatly appreciated. Thanks!