I have a 97 nissan sentra. Today I applied a little more pressure to the brakes than normal. When I did that, the brakes made some sort of grinding, and cracking noise, then the brakes completely went out for about 5 seconds then I tap the brakes and they came back but completely locked up and the breaks will not release. However, I can still go in reverse. What could this be?
Can't tell over a computer. First you must figure out which wheel is locked up. If it's a front one they likely were grinding for a long time and the rotor is ground down to the cooling fins. What's left of the brake pad's metal backing plate can catch on those fins and prevent rotation one way. If a rear wheel locked up, typically something broke without warning and went around the rotating drum until it got caught. That will often release eventually. Either way I wouldn't recommend driving it like that. A tow truck is cheaper than a crash. Either have a mechanic perform a brake inspection, or, if you determine the wheel that's causing the trouble, take the wheel off and post a close-up photo. Look for brake fluid on the ground too inside one of the wheels and tires.
Be aware too that running the brake pedal down to the floor when something gives way like this can damage the master cylinder. Rebuilt replacements usually aren't terribly expensive, but it's easier to be prepared for that possibility ahead of time.