After I changed my iPhone 4s screen the volume buttons don't work?

After putting the phone back together I realized the volume switch and volume buttons don't work. Is this an easy fix? Can I just pop the back cover off and reconnect something

When doing screen replacements there are cables and bands that run right underneath the screen and often these can be pulled out of their sockets, damaged, and even ripped. This may be the case with your phone or the buttons' components could simply be out of place. Only way to find out is, unfortunately, opening it up again and looking. If you can't find the problem then it is a cable/band issue and now you have to fix that too. Sorry if that's the case and good luck as I know these repairs are very challenging.