Mom's knees... Not sure what to do?

Is she a candidate for a Physical Therapist to come and help her?
And if this is "overnight" I would also call the doctor and mention it to determine if there are other problems going on.
If she can not get put of bed at all, if she can not stand, walk is it her knees or something else?
And if she is living alone and is not mobile someone is going to have to stay with her 24/7.
To help with transferring her a Sit-to-Stand might work well or a Hoyer Lift. this will help getting her out of bed, onto the toilet or commode and into a chair. It is safer for her and safer for anyone that is trying to move her.

She may be wheelchair bound now; you will know more after a visit to doctor. If she is then she may need 24/7 care. While she is now helpless and until you can get an appointment you may need to keep 24/7 care with her; you may need to go to ER if this isn't possible. Placement is often easier with the help of the Social Worker at the hospital, so access one right away as she is assessed. After assessment you will know if you are dealing with something that can no longer successfully be surgically addressed, or if rehab is a consideration. Rehabs also have Social Workers to help with placement if needed. I think you are correct. You are entering a new stage.

My mom was doing fine until she suddenly wasn't and she fell because she just didn't have the ability to get up the two steps into her home. Simple PT exercises can do a surprising amount of good, it may be that she has gone too far to recover but I wouldn't give up on mobility without trying.

"...due to her knees"... as in extreme pain? If so, this sounds odd as arthritis or bone-on-bone cartilage loss doesn't happen overnight. Perhaps she fell and has a bad sprain, or tore some ligaments, which can be extremely painful and debilitating. The only way to know for sure is to get her in for xrays. If Granny has any cognitive decline she may be misinterpreting the origin of the pain, it could be her back or who knows what. She needs to be seen and get a definitive diagnosis before anyone decides she no longer mobile.

Is it Arthritis?

My neighbor has been in a wheel chair for a while now due to Arthritis (very painful) in her knees.

She is not a candidate for knee surgery, however, she is getting prepared for a procedure at the hospital.

I don't know all the details but I know they are prepping her to get ready to do some type of injections in her knees. This is to numb the pain so she can walk again. This may be something to look into for your mom.