Can access admin page (now), but not any site pages...yii not found http execption
I am presuming Craft 3 because of composer, but you didn't say. 

To start with look in your craft templates folder. I would suggest that you simply don't have the template that Craft is looking for. 

If you visit the root of your site this is normally templates/index.twig make sure this is present and has no further includes in it (just until you get it working) 

Also try looking in storage/logs/web.log for errors.