Did you check your email sending queue ? Are you receiving any notifications from mantis at all ?
Were the system or user preferences for notifications changed ?

Hi,
How to check email sending queue ? (Note we do not have much traffic on mentis)
No,we do not receiving any notifications'
No,i did not change any preferences except just updated again

How to check email sending queue ?

Use admin tools http://example.com/mantis/admin/email_queue.php
(note, if Mantis has been installed as per recommendation, the admin directory should have been deleted, you might have to temporarily put it back)

except just updated again

What is that supposed to mean ?

Just Updated again means i checked that all privileges set or not for users and then click update preference to ensure preferences.If you wish i can give you log in