My refrigerator was in storage and my water filter broke and only the front half came out of it when I was putting it back in service. I can't get the back half to come out.

There are lots of tricks... but without actually seeing what you have to work with... we can't really offer anything specific. Some might work and others won't even be possible...

Manufacture a make-shift "BIG" screwdriver that will wedge itself onto sides and turn CCW. Look at a new filter where it attaches and determine how to apply proper pressure.