If I understand correctly, your TPV on the HW tank opened. Water pressure is what activates this valve. Excess pressure can be caused by line pressure, excessive temp or both. Also the valve can go bad. The valve is easy to replace; 1)Shut off gas or electric to tank, 2) Drain tank below valve level (most are on top some on side), Remove all pipe to valve, 3) remove valve, 4) Apply teflon tape to threads of new valve, 5) Install new valve, 6)Reinstall all pipes to valve, 7) Refill tank, 8) Power up tank.
Now having said all that. If you feel that you have too much water pressure, you need to have a plumber come out and check the pressure. You may need to install a pressure adjusting valve, a pressure releif tank or both. Excessive water pressure can cause early failure of your plumbing which can become a very expensive proposition. Plus the plumber can determine if you need the new TPV or not and replace it when he does the pressure test, and it's not really that expensive.