Replaced a crank angle sensor in a vn drove like normal then i got back in the car 3 hours later and wouldn't start. might start later though and run normal and stop again what could be wrong?

Check the sensor again. Pull the sensor and check the face and make sure there is no foreign matter stuck to it. Check the postition it is in before removing it. If it's clean then insert it a little bit further into the openning but be careful not to go too far. Since most crank and cam position sensors are inductive any foreign matter on the active area will cause false signals. Metallic shavings in your oil will play havoc on these sensors. Hope this helps.