She could possibly have a UTI, stones,etc. Think she needs to be taken to the Vet for an exam
You don't say if she's still drinking water (Which is very important!)
In a senior dog, this could be many things......Some--serious, some--not so much.....However, any dog without food (Hoping she's still drinking) for 5 days is extremely dangerous......
Like Margot said, this could be a simple Urinary Tract Infection that needs treated quickly.....Prescription Antibiotics will be used.
Or--Diabetes or Kidney Failure-Very serious
When's the last time she had a blood test run? Most senior dogs have them yearly. Has she had one recently & if so, what were the results?
If not, She needs a Vet visit ASAP with a complete blood panel run to look for any underlying conditions......Take a urine sample w/you to check for UTI or protein loss. It will be less money if you can collect your own sample to take.
I wouldn't wait any longer--5 days is long enough & she needs to be seen! Let us know what you find out.......Good luck, Karla