Yes, there is a brake... but on most walk-behind mowers the blade is directly coupled to the crankshaft, so the brake stops the blade by stopping the entire engine. (I believe your mower fits into this category.)
I suggest you disconnect the spark plug wire (for your safety) and then look under the mower and make sure the blade is still tightly coupled to the bottom of the engine. 1) The center bolt could be loose, or 2) you may have sheared a key between the crankshaft and blade mounting hub. Some minor disassembly may be required.