Nexus 4 envelope icon notification disappeared from my mobile notification line

[Android Question]

Hi All,

I have a Nexus 4 mobile phone.

Usually when i get an new email, a envelope icon notification appears on the upper left side of the screen (the notofication area).

few days ago it disappeared.
other notifications still appears (new text msg, new whatup msg and so on).

could you please provide me some instruction how to restore the new email notification to my mobile.

thanks a lot