Hello, I'm Dr. Deb and will do my best to help you today.
Constipation issues can be troublesome for a number of cats although the problem does appear to worsen as they age. If you can't isn't terribly constipated and/or has only had this problem for a few days, then the suggestions I'll provide below may be enough to get things moving again.
1. Hairball laxatives such as Laxatone or Laxaire which are brand names but every pet or grain store will carry these products. I usually suggest 1-2 inches 2-3 times a day.
2. Canned pumpkin (NOT the pie mix) at a dose of 1/4-1/2 tsp per day. Many cats like the taste of it and there are some diets available which contain pumpkin.
3. Over the counter, human Miralax (polylethylene glycol 3350 aka macrogol 3350) at a dose of 1/8th-1/4th tsp daily which can be increased until soft stools are produced.
4. Raw bran flakes (wheat bran, not cereal) in canned food 1/4-1/2 tsp daily.
5. Pediatric glycerin suppositories which can be repeated 2-3 times with several hours in between, if necessary. http://pedia-lax.com/products/liquid-glycerin-suppositories
Just be sure to double check the label to ensure that glycerin in the primary ingredient. Those with phosphate in them can be toxic to cats.
Any and all of the above can be given at one time although diarrhea may result if you do.
If he/she remains constipated and you're unsuccessful in getting her to produce bowel movements within 24 hours or so, then a vet visit may be prudent.
I hope this helps. Deb