Unfortunately for Andersen you really need to purchase a new sash. I would measure the visible glass only and take the measurements to your local LUMBERYARD or WINDOW DEALER(not Home Depot). They will order the sash for you and provide you with detailed instructions for the removal of the old sash as you need to carefully remove the split jamb liner then unplug the plastic piece on the end of the string being careful not to let it fly up and hit the plastic head piece on both sides of the sash. If its the upper sash your replacing you will need to repeat the string removal again. Replacing is done essentially in reverse. I would still ask at the dealer as the have these printed up that will make it much easier to follow.

This is voice of experience. When removing the balance strings,use a pair of vice grips with rubber pads on the jaws to grip the string at the location where it goes into the top of the window before removing it from the sash. Do this on both sides at the same time. These have a lot of tension on them. When you get the sash out hold onto the plastic piece that goes into the sash and release the pliers and SLOWLY let it spool into the head jamb. If these get away from you they will destroy themselves.

If you don't feel confident doing this yourself, let someone who has experience take out the old jamb and put in the new one. Before you put it in stain or paint the interior. A lot easier that way.

Call Andersen. They will directly sell you the sash and send a tech to your house to repair it.

Takes about 20 minutes and it'll set you back about 500 bucks.