Is this a male cat?
How old is the cat?
Is the cat eating and drinking?
Inside or outside cat?
I am not so much worried about the lack of stool, but the lack of urination. This can indicate a serious issue to include enlarged prostate, crystals, stones causing a blockage. The inability to pass urine is extremely dangerous as the bladder can rupture and go septis, causing infection throughout the body leading to imminent death. Please get your Kitty into the Vet asap. This link will go over how dangerous this is : http://www.marvistavet.com/html/body_urinary_blockage.html Joan