It depends. Check your deleted mail folder. If you have a free email provider such as Hotmail or Yahoo and the mail is not there then it is likely gone. If you are using a mail program on your computer and not a free service provider what mail program are you using to access your email?
If an email is deleted in Yahoo mail it is moved into the Trash folder. Check the trash folder for the deleted messages. Once the messages have been purged from the trash folder they are permanently deleted. This is one of the downsides of using a free email provider. To prevent this problem from happening again you can register you own domain name and get an email service that includes a more robust email backup service.