Some random thoughts...
It's possible she may not like the litter you've been using, maybe the scent or texture...have you tried changing it? Then again, she may not like litter at all. You could try an empty box, a box with newspaper or an old towel. Or even puppy training pads. Is she declawed? They often develop litter box issues because it hurts their paws to walk on it. Maybe she doesn't like the box itself...is it covered? Some cats are not comfortable not being able to see all around when going.
Do you have more than one box? She may not want to pee and poop in the same box. Or your other cat might be preventing her from using the box. If you decide to make any of these changes, do them one at a time.
If none of the above applies, I would recommend confining her to a small room, preferably a bathroom with no rugs in the floor. Put her litterbox, food, water and toys in there and be prepared that she may be in there for a week or two.
Recommend Cat Attract litter (or they sell an additive that you can out in your regular litter). Scatter toys around so she has very little space that isn't occupied.
The theory here is that cats typically won't pee where they have to live. If she starts using the box, praise her effusively, give her treats. But you need to ensure the behavior is ingrained so you'll want to leave her in there for a week or so after she is using the box consistently.
Oh...and make sure you clean all of the old pee spots with an enzymatic cleaner to break down the pee and completely remove the scent, otherwise she'll think it's an OK place to go. Get a black light and mark all the spot with a coin and clean them all at the same time. Make sure to really soak the areas with the cleaner, it must soak down as deep as the pee did. Just spraying the surface won't do it.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Holly, Misty & their friend, Jake, the dog.
Onyx, Callie May, Maggie & Kobi forever in my heart.
Yes, I know I need a new signature pic...someday...