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It's possible that this is an automatic reaction to your quota of Yahoo! email messages, or the space used by them, being exceeded or filled up.
How long have you had this Yahoo! email account? do you delete many of the emails from it regularly? and what usage of its alotted space has the account occupied recently?
And also: did you receive a lot of emails that you did not delete, that had (large) attached files, such as photos?
Then, unless you shared the password ***** your account, automatic culling of emails from your account is unfortunately rather probable under the circumstances, I'm sorry to say.
OK. Then try logging in to your email account with Yahoo! by way of a web browser on your computer - that access may differ and still show all your older emails present, if you have been accessing your email through an email program such as MS Outlook.
With what program do you normally access your email? I'm wondering whether there might be some trace in it of the apparently lost emails.
I don't know what else to say, except: Contact Yahoo! support if you like, to ask basically "what happened?" and they might possibly have any number of answers to this problem. Or even, doubtfully, they might restore the missing emails.