Constipation for weeks warrants a physician's visit.
A suppository or enema might be a good idea to get out what's already there. A large amount of magnesium citrate (a LARGE amount) I believe is used for colonoscopy prep, so enough of it will clean you out, although it would probably be miserable. Make sure to drink plenty of fluids as you continue, and add more fiber to your diet. Eating the actual prunes with fluid is probably better than prune juice. You can also start adding a fiber supplement like metamucil or benefiber, or ask your doctor about getting miralax. Any fruits other than bananas and apples are good for keeping things soft.
Just make sure you keep on top of it from now on.
Try magnesium citrate. You can also use Metamucil, which comes as a powder you mix with water or as capsules. You can also get things started by using glycerine suppositories.
make sure that you are keeping yourself very hydrated. i know that coffee helps some people too, you could try that. also make sure that you are eating a lot of fiber. like broccoli, nuts and grains.