I found my cat panting and she is not able to stand. Her back legs seem weak. I put her in a basket with water because it is after hours at the vet. Is this a stroke?

It could be a number of things.She may have got into something toxic,or some sort of health problem.If you can not take her to a vet,keep her relaxed and calm,and hopefully she will show improvement.I would take her first thing Monday morning and have the vet run a health profile on her blood to see what is exactly going on.The bloodwork takes about 20 minutes to run,and it may give you an answer about her sudden decline.Could she have been hit by a car?Let us know how she is doing.