If your car is stuck in neutral, there are a few things you can do to try and get it out of that gear. First, check to see if the shift lever is actually in the neutral position. If it's not, gently move it until it is. Sometimes the shifter can become dislodged and end up between gears, so this is a good first step.
If the shift lever is in fact in neutral, then the next thing to check is the transmission itself. Make sure that there is no debris or obstructions blocking the movement of the gears. If everything looks clear, then you may need to have your transmission checked or replaced by a professional.