You do not have access to Microsoft Dynamics NAV. Verify that you have been set up as a valid user in Microsoft Dynamics NAV.

Dynamicsnav 2015 windows client error when opened

“You do not have access to Microsoft Dynamics NAV. Verify that you have been set up as a valid user in Microsoft Dynamics NAV.”

The user was working fine a day before

Have you change any permission of user on SQL Database. ?

As well as Check whether user is added in database or not

it was working fine almost for five months it just happened all of a sudden.

Check if the user has an expiration date in NAV.