Sometimes incoming SMS fails to reach your salesforce org. The error thrown is:
This means that the user who has set the Oauth for your account is currently inactive. Users only become inactive if someone unchecked the Active box on their User account. You can revert the same by following below steps:
Click Your Name > Setup > Manage Users > Users > Edit (next to user's name)
Check the Active Box and Save.
This should resolve the issue.