Thanks for answering.
Using gmail through Mozilla Firefox. Have checked, and all my incoming stuff has disappeared from gmail, AND Mozilla. Pretty sure I didn't delete these mails, and missing a good number of fairly vital messages.
Clutching at straws here, but the last program I used was iTunes (and even that DIDN'T send me the new ID as it promised).Don't use iTunes often and forgot my ID. Have I got a virus, or just senility?