Checked if one of your scheduled jobs for domain index has failed or is in status Warning. 
check background jobs. 

I cannot find any jobs for domain index. Can you please advise if any standard Job for this.

Please try the following actions to make sure all the domain indexes are in valid state. 

1.    Go to Solution Manager -> User Interface -> Application Search → Search domains.
2.    Find the corresponding search domain related to Documents  (Eg: DocumentObjectConnection, DocumentContent, DocumentAttributes)and RMB -> Show Details
3.    Normally this screen should have a green flag saying that all the documents are synchronized. If any of your search domains has warnings or errors, try dropping the index and recreating it. 
4.    RMB -> Drop Search Index
5.    Next RMB -> Schedule Domain Activation
6.    Now you will get a new option to “Enable Schedule”. Select this and go to scheduled task screen.
7.    Now mark the schedule “Activated” and RMB -> Run. This will create a background job to rebuild the index.
8.    Once this background job is “ready”, the index should have been rebuilt. Next check whether there are any other similar indexes having the same issue. So, you can continue the process and rebuild all. 

Please note that the time taken to complete the background job depends on the number of documents in your application. We strongly recommend to schedule the background job to run during a weekend so it will not interfere with your day to day business activities.

Thank you for providing steps. We have rebuilt the Index through database script :

SQL> drop INDEX "IFSAPP"."FND_USER_TIX" force;

Index dropped.

SQL> CREATE INDEX "IFSAPP"."FND_USER_TIX" ON "IFSAPP"."FND_USER_TAB" ("TEXT_ID$")  INDEXTYPE IS "CTXSYS"."CONTEXT" online;

Index created.

SQL> alter index IFSAPP.FND_USER_TIX rebuild online;

Index altered.

SQL>