Why cann't I Look for a file using Cortana?

Hello,

In the past I used Cortana to look for a file by typing its name or portion of it and Cortana found it for me. Now Cortana doesn't do it? What am I doing wrong? Cortana just expects me to speak and I don't have a microphone installed. How can I make it work. I basically use Cortana to find files.
Please help