Why would a car not be going into gear at all?

If your transmission is a manual one, chances are, that you have a hydraulic clutch cylinder that has gone out. The sure fire symptom of this is, you have the car's engine running, you are ready to put it into gear, whether it's a forward gear or reverse. Nothing happens, you depress the clutch and you can't get it to go into gear. Your cylinder is bad. You don't specify what ype of vehicle it is or even if it's a manual, but the automatics usually go into gear but they will have other problems like slippage. Clutch cylinders use brake fluid, dot 3 or dot 4 and can be bled like the brakes on your car. So look for a vertical tube on the drivers side of the engine, close to where your brake fluid master cylinder is. It really depends on model and year to give better diagnosis. But, if automatic, fluid is full, and there is no movement (not even a hint) the trans is bad. A faulty torque convertor or bad pump will do this. Dan the tranny man.