Go to a Pharmacy and get a bottle of Magnesium Citrate and a box of Fleet Enemas. Drink the bottle of Magnesium Citrate...the whole bottle.) It's gross so drink it chilled if you can, and have water on hand. Drink it quickly and get it over with. Then wait. You should start getting cleaned out about six to ten hours after that. You are going to go to the bathroom ALOT...so make sure you don't have anywhere to be.
Or....administer one of the fleet enemas to yourself by following the instructions on the box. This will cause you to immediately go.
Neither of these things is a long term fix. They are just to get you going and to give you some relief. If you continue to have trouble, the next step is to modify your diet so that you are getting enough fiber and keep drinking the water. You may just go back to normal (I hope! IBS is horrible!) But if you don't, then start looking at your diet and start exercising and you might want to try Miralax or probiotics or enzymes.