Cat with paralyzed back legs.?

My cat "Princess Renee" is 15 years old August 6th. Just last night she was perfectly healthy and active. However, i was woken up this morning to my boyfriend telling me something was wrong. She could not feel or move her hind legs or her tail. She is still alive but is now hiding from me and her breathing is getting heavier. I have literally grown up with this cat and think of her as a sister. What are my options? What has happened to her? Thank you.