Search not finding contacts

Search not finding contacts

Hi

We have suddenly found that leads / contacts etc which we worked on recently are not appearing in the search results.

But if we load the same client as a lead, and convert the lead to a new potential, CRM picks up the contact. 

So it seems that the search function is not finding the contact, although the data is on CRM.

Is anyone else having this problem? 
Is it a general issue that Zoho is aware of?