It is possible that your rat had a stroke. The easiest way to tell is if there is paralysis on one side of the body (very little movement, if any). It can go away on its own, or with the help of a medicine like prednisone or medrol. If it's the back half of the rattie that's "paralyzed" it's often caused by some degeneration of the spinal chord. It's common in elderly rats. Steroids don't always fix that, but the rat usually adapts just fine. There are two excellent articles on rat paralysis here:
<http://www.rmca.org/Articles/paralysis.htm>
<http://www.rmca.org/Articles/oldrats.htm>

If, however, you can find any swelling, it is possible that the rattie has an injury, either a broken bone or a pulled muscle. The swelling doesn't show up immediately, so check the rat over again. There's not a lot you can do for this (rats won't wear a splint on a broken bone, for instance), but these articles on Nursing Care
<http://www.geocities.com/nswfrmc/article8.html>
<http://www.rmca.org/Articles/nurse.htm>

can help you at least make the rattie comfortable. It's also a good idea to get a vet's opinion on what the condition is if there are no signs of improvement after a day, or looks like the rattie is in pain. If you don't already have a vet, we have links to some that have worked well with rattie patients:
<http://www.rmca.org/Resources/vets.htm>
<http://www.ratfanclub.org/vetref.html>

If you have determined that your rat had a stroke, play with it! Move its legs and pet it a lot. Try to touch it and carry it places, because physical rehabilitation has been shown to help speed up recovery times and improve quality of life for stroke victims.