Search function in emails not working

I am unable to search emails - can you please resolve this quickly. Despite searching by specific email addresses (some where I have received probably hundreds of emails) it say no results match the search criteria, similarly if I search by subject line or keyword I get no results. How I can fix this infuriating problem, which is massively hampering my ability to work. It is truly impossible to search manually for messages over many months to find the info I need. Many thanks.