Last Sunday my client was unable to login to the CRM and was showing below error :

HTTP Error 401 - Unauthorized Access is denied.

I checked and found this error is of claims based authentication or IFD. 

It was working fine from past 90 days but i just followed the below Microsoft article and error was resolved.

Cause mentioned  :  
The Token-signing certificate and Token-Decrypting certificate in ADFS will automatically be renewed by the Auto Certificate Rollover feature because these certificates reach their expiration date.

Microsoft KB Article : Click Here