Smack the magazine on the floor plate to make sure it is seated correctly.
Rack the action to clear out the old ammo that might be causing the failure.
Roll the rifle to the side as you rack the action to let the rounds that potentially caused the failure to fall out to the ground. You should be able to do this and get back on target and pull the trigger again without looking. If the rifle still doesn't go bang, you will need to remove the magazine from the rifle, inspect the action to make sure it is clear of ammo and try again.