Any vomiting or retching? Has she gotten into anything that you know of?
If it is just diarrhea, you can try a bland diet (boiled boneless chicken breast with boiled white rice). But if she is not eating or drinking and it has been 24 hours, she should be examined by a vet. You don't want to wait too long on anything more serious or have her get dehydrated with all of that fluid loss.
Also to rule-out causes for diarrhea other than just 'gastroenteritits' or upset stomach, you may want to submit a fecal sample to your vet to look for parasites. There are some over the counter human medications you can give for upset stomach, but I am not comfortable giving doses over the internet. Your veterinarian may be able to recommend some if you give him/her a call.