Not being able to go to the toilet is a painful and disturbing experience, and if it has never happened to you before then it can be hard to know what to do. The worst thing you can do is panic, often it is only a temporary thing and if you have only had trouble for a few days then it might clear itself up as your body was just struggling to cope with something, you shouldn't start drinking loads of senna tea too soon.
If you definitely haven't been able to go for 3 days or more then the first course of action is to see your doctor, just in case something is wrong which is causing the problem. Once you've done that you should know if there is some medical problem that caused it, or if it is just a change in your lifestyle instead which you need to fix or let your body adjust too.
You will probably want to start on a course of laxatives to clear the obstruction. I recommended trying the type which soften your stools first, if you use the pills which irritate the lining of your bowel and try to force your body to go, then that can be really painful if you haven't softened things first to make them easier to pass. You don't have to take pills though, the osmotic laxatives usually also come in liquid form, and the other kind are usually just senna in pill form anyway, so for a few bucks you can buy the leaves straight off Amazon and steep them in water for a few minutes to make a tea which will have the same effect.