If your cat who did use the litter box starts urinating elsewhere, there could be a couple of reasons for this behavior.
Some cats refuse to use a litter box until it’s clean. Other cats don’t like sharing a litter box with another cat. Have there been any changes in routine of the household that could be upsetting the cat? Urinating outside the litter box could be an indication of illness such as a UTI (urinary tract infection). If your cat is urinating in inappropriate places, s/he should be examined by the vet to rule out any health problems. It’s important not to ignore this behavior just in case it is a medical condition.
In order to discover where that awful odor is coming from, purchase a black light. Using it after dark with the lights out, try to sniff out where it’s coming from. A bright green color will indicate a stain.
If the urine is wet, use dry towels, paper or cloth, to blot up as much as you can. Wet or dry, using one of the many enzymatic cleaners on the market, saturate the area – carpet, upholstery, drapes and so on. Allow the area to dry. You may need more than one application to get rid of the odor. Follow the directions on the bottle and you’ll get very good results.
If the surface kitty has urinated on is a wood, linoleum or tile floor, use your regular cleaners – they’ll do the job. Use your washing machine for any items that works for.
If the problem persists, consult with your vet and/or an animal behaviorist.