Assuming you're speaking of the actual Google Notifier application (there's a 3rd party app called "Gmail Notifier") then it's probably because Google dropped support for the Google Notifier at the end of January (2014).
Basically, they suggest you use the Chrome browser instead.
Google Notifier Beta is no longer supported. If you want to continue
to receive notifications, you can use any of the following
alternatives to Google Notifier Beta using the Chrome browser...
...Google thought the same thing, and today quietly discontinued the free tool, which has been around
since 2005. Users looking to install the app are now being told by
Google to simply install Chrome, which has desktop notifications built
Notice sent to users by Google a couple weeks in advance:
Important Announcement on Google Notifier Beta
We are writing to let
you know about an important change to Google Notifier Beta. Starting
on January 31, 2014, Google Notifier Beta will no longer be supported,
meaning the app will no longer show recent emails and calendar events.
Since the Google Notifier Beta launched in 2005, a lot has changed.
Smart phones can now notify us of new messages wherever we are, and
improvements to web technology enable similar features to be built
right into the browser.
If you want to continue to receive notifications, you can use any of
the following alternatives to Google Notifier Beta using the Chrome
To see the number of unread messages in your inbox at a glance,
install the Gmail Checker Chrome app. To preview new messages on your
desktop, go to Gmail's settings and enable Desktop Notifications. To
get Calendar notifications on the desktop, go to Calendar's settings
and enable Show floating desktop notifications. You can uninstall the
Google Notifier Beta by following these instructions.
-- The Gmail and Calendar Teams