Viking Husqvarna model 6440 gets stuck in reverse

There are numerous things that can cause this, some very simple, some less so.
1. Fortunatly, the most common one is also the simplest to cure. When you turn the selector (upper left) knob between stitches, one of the things that is happening when you feel the resistance is that it is lifting the followers off the camstack so that it can move to another stitch. When the camstack is normally doing its job, lifting the feed cam follower is what puts the mechanism in reverse. So, while the selector is between stitches, the machine is temporarily in reverse. Also, the dots where the dial is to be placed to change the camstacks is between stitches, so if the dial is on these dots or anywhere else except snapped into a stitch, you will get nothing but reverse.
2. The buttonhole mechanism is stuck in the reverse portion of the buttonhole. Usually if this is the case, the needle position will also be stuck on the left side, and the width control /buttonhole control will not function correctly. Usually you can correct this yourself and I'll explain how in a bit. It is usually caused by lack of use of the buttonhole mechanism.
3. Either the arm that connects the camstack to the stitch length block or the block itself is stuck. Both these parts are spring loadedto bring them in the forward position, so long periods of rest are a contributing factor.
All of these causes except for the first one are because of the mechanism not being exercised frequently enough. These older Vikings do not like to sit still very longYou should fully exercise all the knobs at every sitting.
Ok, try this fix. And this is to free up the mechanisms that are probaby stuck from non use and old oil and grime buildup. Take your seamformer (cam) out of the back. Find a hairdryer and blow some heat in there for awhile. This will soften up any hard oil or grease that may be causing the mechanisms to stick. Then if you can it would be best to have what is called TRI-FLOW oil (the BEST) and spray or drop quite a bit into there covering the mechanisms and let it soak in. You might try a good sewing machine oil soaking or WD-40 in small amounts. Little by little test your knobs, slowly working whatever is stuck lose. Never force the issue. Small amounts of movement little by little will eventually free it up.
Good luck. Hope this helps. If this does not help you may have to take it to a technician and hopefully you won't find out that your take up slide isn't broken which is a major job.

You can find about 8 videos on how to solve these common Husqvarna Viking problems.