The mail notification in the Individual lead page disappears.

The mail notification in the Individual lead page disappears.

Hi

We have been performing numerous Mass email schedule to out leads over the past few month now. However the sent emails notification along with the mail both disappeared from all the leads accounts. If you van see the screenshot that i have attached, the notification just disappeared without a warning. Can you please assist me as to how I can get these mails back as they were regarding to my business plans and I need to retrieve them as soon as possible. 

Please revert back with assistance as soon as possible. 
Also let me know how I can get these back.

Regards,
Adarsh