Try force quitting TechTool Pro, by holding down Command Option Escape. That might let you get back to the Finder and restart.
You can power down the Macintosh using the power switch. Exactly how you do that depends on what model you have. It you have to, you can pull the plug. There would be damage to the disk directory only if the freeze took place while the present disk directory was being replaced, which happens only if you click the Replace button in the Technical Comparison window.