Watch your kitty. If your cat has not peed or pooped in 2 or more days your kitty could be stopped up. This is pretty common with cats. You just need to take your kitty into your vet so they can express its bladder or bowel. If not your cat could get into so much pain, it would really hurt your kitty.
Unable to urinate would mean a blocked urethra, and this is dangerous . Mainly happens in overweight males, neutered or not. Gastric block is unable to deficate. Meaning your cat could have eaten a foreign object and will have to be more than likely surgically removed. Expressing the bladder is not a cure, and if the urethra is blocked, it is not an option. If eating and drinking stops, there will be no urinating/defecating. So being blocked in one way or another is ruled out. Your cat could have a serious medical condition that could be an emergency, so do take your cat to a veterinarian as soon as possible.
Yes it is a sign something is wrong. You are saying that it didn't eat, as in it is eating now. I'm not clear with your situation. Diagnosis cannot be made on the web-site. I'm sure you're not searching for an accurate answer, just advice. Lets say your cat is eating now. Is this strictly indoors? If not, it could be eating outdoors at the neighbors maybe? The cat may have eaten something that made him or her feel ill for a day or so. (highly unlikely, unless a low toxic plant or other source) It could also be parasites. The bottom line is, take your cat the to vet if this is still a problem. It could be a number of things, some could be life threatening. I wish you luck and hope your kitty is not sick!