Thanks for the information.
I would be more inclined to think she may have some sort of internal injury since she was outdoors. She may have been hit by a car, kicked by a mean person, or fallen from somewhere high.
When a cat is lethargic and not eating, that is an indication of pain. Sometimes they will also hide, or hiss at you when you approach, and constant purring can also indicate pain or illness.
Since there is no vomitting, no drooling, and she is not stubling around, I do not think she was poisoned, though anything is possible since there is no way to know what she did outdoors.
If her gums are white, this is an indication of shock and she should be rushed to the vet immediately.
Since she is only drinking we do need to get some food in her.
The danger in not eating is that the liver can begin to break down fat reserves which it is not very efficient at doing, causing it to become clogged for lack of a better word. This condition is called fatty liver syndrome and is dangerous. It is always secondary to another injury or illness and usually worse than the original condition. Try the smelliest foods you can, even tuna. If that still does not work, we need to force her.
If your kitty is not diabetic, get a bottle of unflavored pedialyte, a 1 cc dropper and canned cat food or baby food chicken or turkey with no onion or garlic. Give one dropper of pedialyte every 10 minutes for one hour. Than give one dropper of food mixed with plain water every 10 minutes for another hour. Wait two hours and repeat both. Once your kitty is feeling better (nourished and hydrated) she should begin to lick it up on her own.
I urge you to have her seen today, since tomorrow is Sunday and vets are closed. Most vets have some Saturday hours, and if you get her in today, it will accomplish 2 things...the first obviously, is helping your girl before allowing it to progress into something more serious, but the second thing is you will also avoid possibly having to bring her to an ER vet which can be costly.
I hope this helps you, please let me know what happens, I will be thinking of you!