My rat's cagemate (maybe sister) died 3 days ago. Until today, she seemed alright about it. She was still coming to the bars when I was near the cage, trying to climb onto my shoulder, climbing around, and running on the wheel. But today, when I come in I find her curled up in her nest. If I come close she'll just look up at me and go back to sleep. She did go to get the food I left her (on another level of the cage) and I even heard her running a bit from the other room.
Could she be depressed? If she is, is there anything I can do to make her feel better? (Besides trying to spend more time with her, of course. And I'm planning on getting her a new cagemate as soon as possible.)

She sounds depressed, and if she lost her sister I don't blame her. Be understanding towards her and keep talking to her, let her know that everything will be alright. Spend more time with her to make her happy; take her out of the cage to play and give her treats and plenty of TLC. A new cagemate might help her out too. Good luck and I hope she feels better soon.

Yes, your rat is going through depression, but who can blame her? In the mean time while she is waiting for a new buddy spend more time with her and make her a feel a little better by giving her treats and comforting her. Most definitely get her a new cage mate. Most animals tend to live better with other animals of their kind, but like I said only MOST animals that is. Some animals preffer being alone. Take care! Best wishes are with you!

Knowledge and experience

It does sound like shes a little depressed. Theres not much you can do. She should be fine until you get her a buddy. Just get her a buddy asap and monitor her eating and weight. Some rats will starve themselves to death when their buddies die. pets question and answers,www.5d2d.com
Please adopt. You can look on petfinder.com and at shelters and rescues.

If you have enough answers you have to choose a best answer to close the question. Pick me! just kidding. You got some good answers. The people who answered deserve credit.

i'm not sure about introducing a new rat never tried it but if u have the room and money buy a new rat and another cage and put them next to each other, i had 2 rats in seperate cages one died a few yrs later and the other rat was withdrawn like urs she would not come out of bed i got a new rat in the cage next to her and she was back to normal again, i really do think she was depressed and grieving even though they didn't live together in the same cage, i've had rats together but none of them have bothered when their partner died