He was 18. I found him on the floor of my study on his side, with his paws in 'running' position. He always comes down to be fed in the morning, at noon, at night. He always used to wake me up in the morning. This morning his sister - and litter-mate: they have never been separated, since birth...
Does your cat have kidney disease? It's very common. If your cat has kidney disease there are things you can do (my two senior cats have it), but first you have to know what you're dealing with. The concentrated urine suggests a UTI and/or kidney problem.