If you deleted an important email from your Gmail account and realized that you need it, you can easily retrieve it using tools and methods that Google themselves provide online.
Quick answer box:
How to recover deleted mail in Gmail?
How to retrieve deleted Gmail emails? 
Here in this post, we show you how to retrieve emails you accidentally deleted within 30 days of deleting them from your account.
These emails actually aren’t deleted as such and just end up in your “Trash or Bin” folder. You can easily restore access to these from your “Trash or Bin” folder by the following methods.
1. Searching mail to retrieve
You may just do a simple search for the email you misplaced by typing in something related to the email, like the sender’s name, the subject etc. Once this is done, Google as usual serves up some results based on the search term and you can see if your email is in the list. If it is, just select it and click on the remove “Trash or bin” tag and the email will be restored along with the attachments if any.
2. Search for Trash or Bin folder
if you can’t find your email by the above method, just scroll down on the left most pane to the “Trash or Bin” tag folder and you can see for your deleted email in a chronological order. Once you find it, open it up and remove the tag by clicking the button as before. You’re done.
Untag Bin or Trash at the top
Once you delete an email, Google doesn’t delete it immediately from their server. They keep it on for 30 days at least. Once these 30 days are over, you might need to contact Google’s customer service to help you.