Take the end cap off, take out the batteries and look down inside the laser with a flashlight.
Is the spring standing up straight or is it leaning over to the side or touching the inside wall of the laser?
If so, it is shorting out against the inner wall of the laser which will cause the laser not to work at all.
If you are the DIY type, take a long shank screwdriver with about a 1/4 inch wide blade. Slide it between the casing wall
and the spring about 3/4 of the way down the length of the spring and push it towards the center
of the laser. Do not let the spring snap back as you will have to start all over again.
Once the spring is straight, it will stay straight.
Another fix is to put only one layer of clear, thin scotch tape around the negative end of the first battery with
about 1/8th of an inch of the tape sticking out. This makes a collar around the battery that will stop the spring
from touching the side of the laser.
This is very important - As you start to slide it into the laser, make sure it is not too snug and it is
not going to get stuck.