This same situation literally just happened to me! Complete with female Holland lop who used to have good litter box habits
By the time I checked her butt, my bunny had little mats and tangles all around her butt and on her back feet. I didn’t feel comfortable trying to cut them out myself and wasn’t sure if a shave would be necessary, so I took her to the vet. They clipped out a few mats but wanted to avoid shaving her because having fur protects against urine scald. They suggested I brush her butt regularly to prevent it from getting worse and encourage the remaining mats to grow out. It’s slowly working, but you can imagine how much she likes that…
As far as getting her to go in her litter box again, I don’t have a ton of suggestions. I do know that my bunnies tend to pee on the cage floor when the litter box and/or cage is dirty, so I’ve been trying to keep everything cleaner than usual and that’s been helping some. But in the meantime, I’ve been lining their cage with a towel – it absorbs the liquid so no one can sit in a pee puddle
The towel has helped a lot, since Mimi’s butt doesn’t get wet anymore. It’s still a little yellow, but she doesn’t seem to mind and as her fur grows out it should slowly be replaced by clean fur.
Good luck!