I've raised my cat from a kitten, she was maybe 3 or 4 weeks old. I've been by her constantly. I am a stay at home mom so I really mean constantly. I carried her around the house with me, put her in my lap and pet her. She would come to me all the time. She's never changed. We moved into my husband's mom's bottom floor because she has dementia. She stayed on the bottom floor with us and was the exact same cat. Then one day when my husband was drunk, he took her up there. She stayed up there for days and when she came back down she was a completely different cat. She runs when I try and pet her, even if she comes to me directly, she growls when I pick her up, she stopped responding when I call her name. I feel like I lost my baby girl and it's breaking my heart. He took her up there and now my baby isn't herself. What do I do? Did I lose her forever?
I know it's hard, but you must consider the possibility that she was abused and/or traumatized.
She stayed up there for days and when she came back down she was a completely different cat.
Why would she stay in a place she doesn't know for days? A place where she had no food, water or litter box? The most probable scenarios are that she was locked there and was conciously stopped from returning home, or that she was so stressed that she hid away for days until she finally managed to move back down. The latter also happens to some cats after a move to a new home, so you can asses how likely it is.
Neither drunk nor demented people are thinking clearly and it's possible they do something meaning well and don't realize that the cat is absolutely stressed out and traumatized by that.
That your husband claims
He completely denies that he had anything to do with it and that I should just get rid of her.
is a big fat lie and his way of denying responsibility for his actions (not only with the cat, but probably his drinking in the first place). Unfortunately that's not unusual for people with alcohol problems.
What do I do? Did I lose her forever?
There's a good chance that she can relax again, but it will take time. You need to regain her trust by not touching her at all when she signals you that she doesn't want to be touched. Lay or hold your hand close to her and let her decide if she wants to approach you. If she doesn't, you must accept her wish.
I would put her litter box (or an additional litter box) in the spot where she pees now. She might not feel safe enough to use her usual litter box. You might also consider using a special enzymatic cleaner to get rid of the smell.
To help her feel safer in general, you can offer her more hiding places. That can be as easy as putting an old pillow or towel into a nook between a wardrobe or cupboard and the wall. Think of any places she could fit in where it would be impossible to snatch her up. Cats also love places above ground where they can observe the whole room. So giving her a dedicated place on top of a cabinet, shelf, windowsill or simlar may also work.
You can also move her food and water bowls to the place where she likes to spend the most time. Please don't put them directly next to the litter box, cats actively avoid drinking water from the place where they defecate.