Answer

Hello and thanks for researching this very important question!

There are some things to try at home, but if you do not find clumps of urine in her box (or anywhere else in the house!) today, she may be having a medical emergency (such as a small bit of bladder crystal blocking her urinary tract). Cats that do not produce any urine at all need to have their kidneys evaluated.
Emergency Financial Aid help here.

Canned cat food does help keep the digestive organs moving better with cats, but you can try offering some plain milk...milk often gives "normally" moving cats diarrhea, and can "unclog" one that is constipated.

Pedialyte is fine to offer and can help correct dehydration...mix 25% with her water and change daily.

Canned pumpkin (NOT pie filling--just plain pumkin in a can) can also help get some fiber into the bowel.

If her colon has backed up with very dry stool over the past week or several weeks, she may not be able to get things going again on her own...and again, will need some emergency attention to get cleaned out and back into balance. Older cats can develop a syndrome that results in the colon stretching out of shape...medicines and dietary adjustments are used to support these pets.
Read about Feline Constipation Syndrome here.

If you need additional support at this time, please "Reply", otherwise I thank you in advance for your "Accept", and hope she's feeling better soon.