I've bought a Waring Pro 855 Meat Grinder on Craigslist, and while the motor works, the grinding screw does not engage either in forward or reverse.

That is a screw determined in the feeding tube. Its work is to seize the meat pushed into the tube and handed it over to the cutting blade for grinding.Click here for more info Category 1 HP Meat Grinder

I used to make ground meat and sausage by myself. So meat grinder is a must-have for me. Normally I use KitchenAid meat grinder attachments. It's multifunctional but I need something more portable sometimes. Finally, I got this grinder. Now I can do my job anywhere in the kitchen. Believe me, you won't want to move a stand mixer frequently. This unit is solid. The motor is very powerful and works like a charm. 5 lbs ground meat is done in minutes. Normally I froze meats for 2 hours before grinding. That helps a lot. There is 3 size grinding discs for different needs. The medium is the one I like the most. It also comes with sausage attachments. Everything I need is included. I have nothing to complain, great buy.Click here for more info Best Meat Grinder In 2018 Quick Look At The Best Meat Grinders

The problem is due to the gears that have been damaged and if not then is the motor, many times the first problem leads to the second and the manufacture does not give any of the motor replacing parts only the extrenal parts which you find at the link Also here is the manual of your grinder at which you can remove all external parts and try to rotate the motor forward or reverse if not any reactions then we sugest for a replacement apliance.

I've long ago lost the manual that came with my grinder (I actually don't remember if it even had one), but I don't think it listed grinding times. You just grind a few seconds and check, then grind a little more if it's still too coarse. Unlike a burr grinder which is adjustable, in a blade grinder the same grind time may result in different consistencies as you change beans. By now I've just learned how long to hold the button for the different beans I buy, and the first few times I try a new blend or varietal I have to adjust the time. I think you'll just need to experiment and then you'll know what works best for your tastes.

http://www.waringproducts.ca/en/pdf/ibs/mg100nc_en.pdf

If you remove the meat grinder, does the mixer still work? It could be that the meat grinder is jammed.

You might have also blown a breaker. Have you tried plugging a lamp or something else into the outlet to make sure it's still working?

Firstly I would check it is fully engaged. If it is, remove the food grinder and look into the hole. You should be able to see a square hole inside. Turn on the mixer, the square hole should spin, if it does not, one of the gears inside the head has most likely broken. To repair this you will need to take the head apart.

The other possibility is that I misread your problem and that the grinder is turning, but the meat wont go through the plates at the end. This is a common issue which happens if the blade is not put in the end of the grinder. The blade is shaped like a cross and cuts the meat when it reaches the end of the screw.