OWA - Users cannot search

Good Morning,

This is my first ever post in community. Hope all are well.

I have a couple of users that search function in OWA, has stopped working. If they use the search box at top of OWA. No search results are displayed, regardless of the search target is changed to inbox or any other option in the search target in the box infront of search field?

If I open another mailbox via OWA and open both users mailboxes. I can search the Users mailboxes without issue.

Can anyone please advise if there is a know fix or fixes to try?

I need to get this issue resolved asap as users escalated tickets in ServiceNow via help desk!

Any help much appreciated.

Many thanks 

Blitz