I just finished watching a really interesting show this afternoon about cats on Animal Planet. They showed how an experiment was done with a bunch of cats. They didn't know each other and were put into a room together. They were all very stressed and some were being aggressive and others were just trying to find a place to hide. They repeated the experiment except the next time they played soothing music prior to the meeting and during the meeting. They were amazed by the results and showed that the cats were not aggressive this time around and some were even engaging in grooming each other.
Maybe you could begin to put the two cats in a neutral room. Not the upstairs bedroom and not the place the new cat hangs out downstairs but a different room that isn't special to either one of them. You could try the music thing. At some point, they should meet and it might take a few visitations before they accept each other. Worth a try.
I also think Mary had a very good point about getting your cat to the vet as there may be something else going on if she is no longer grooming herself. And, as she also mentioned, there are some meds out there like Clomicalm that might be able to help your Maine Coon get along with the new cat.
And, as mentioned by others, visiting with your Maine Coon and reminding her that she is important and loved will hopefully help her to feel less 'snubbed'. Maybe you could make a point of spoiling her rotten for a few weeks and see if it helps turn her around. Toys, treats and rubbies with lots of quality time.
Please keep us updated and let us know if the situation improves.