If the axle did not snap into place all the way it is possible it popped out, not all the way just enough to where the splines can't engage the diff assy. Remove the axle and check the end of the splines to see if they are damaged. If they are damaged you may need to get a new axle or take it someone who can repair it. This is the most common mistake of all people who do repairs themselves so don't feel bad or get pissed off. It is a very easy fix. Hope I've helped.