My 15 Year Old Terrier Mix Has Been Fine Till

My 15 year old terrier mix has been fine till tonight. He went for a 1/2 mile walk this evening as usual and ate his dinner fine. went out earlier tonight to urinate as usual. When I went to take him 30 min. ago before bed, he couldn't get down the steps, didn't urinate and I picked him up to carry him to his bed. He's just laying there. I know he's old, but he seemed fine just this evening