My dog hasn't pooped in days - what could be causing this?
These symptoms could be the result of a whole multitude of problems with your dog. You should certainly take him to see your local veterinarian, who will be able to undertake a far more detailed analysis of the situation than is possible here.
The vet should also be able to make a quick diagnosis and hopefully start treatment for the problem right away.
I can only speculate that your dog is suffering from some sort of digestive problem, which may be causing a blockage somewhere. This will cause your pet a great deal of discomfort.
Fortunately, most problems like these can be treated rather quickly. The vet will administer medication that will hopefully make your dog poop. If this doesn't return your pet to its regular routine, then a more detailed examination may have to be undertaken.