Almost 99% of the time problems with Agent Ransack not finding files is down to forgotten criteria (e.g. a date/time restriction). Because Agent Ransack starts-up loading the previous search it can be easy to forget that you'd, for example, unchecked the 'Sub-folders' check box.
Simple Solution
If you've just installed the product please reboot. Then before trying anything else do a new blank search via the menu option:
File->New->Blank Search

IMPORTANT: Do not type anything into the criteria fields, no File Name, no Containing Text. Now click the Start button and see if any files are shown. If nothing is found go to the next option...
Advanced Solution
If a new Blank Search didn't work the next step is to reset the configuration settings, which are normally stored in:
C:\Users\<user>\AppData\Roaming\Mythicsoft\AgentRansack\config
With Agent Ransack closed first delete the file:
history.xml
Try a simple search and if that doesn't solve the problem close Agent Ransack again and delete the file:
config.xml
If that still doesn't sort the problem out contact tech support or post another question here.