Hopefully, it will be something simple, like, you're taking phenypropanolamine or pseudophedrine for a cold. If you already have essential hypertension, then anything you do or take that tends to elevate blood pressure will tend to aggravate your already high blood pressure and will make it harder for your antihypertensive medication to do its job. So I still say, look for the simple stuff first, even though I do respect ed34. If there isn't a simple answer, then start ruling out the more complicated stuff.
Not to dispute ed34, because he knows what he is talking about, but it only takes one or two cups of regular coffee or a moderate amount of salty food to shoot my blood pressure through the roof -- but then again, I do have something else wrong with me. I have a bad heart valve. So yeah, there are other things that can be checked out. Kidneys and heart valves are two of them. I don't know what all else.
If you are not drinking gallons of strong coffee and consuming tons of salt in your diet and suffering unusually high stress, I would get your Doctor to run some blood tests for kidney function etc. What is your average heart rate at rest?
I wonder if you are taking any other medication or even nonprescription drugs like caffeine that may have caused your blood pressure to jump up. With a truly sudden elevation, I would be looking for something else in my life that changed around the same time or right before my blood pressure went up. If you are female, estrogen replacement therapy can make your blood pressure go up; estrogen acts like a salt. Which brings us to dietary changes, are you suddenly getting a lot more salt in your diet. Also, some herbs, like licorice, can trigger a rise in blood pressure. Stress, anxiety, or physical pain can do the same thing. You have a detective's job to do. Something is working against your antihypertensive medication. If the medication was effective in the past, what has changed in your life?