Could be any number of things. Linkage, bad/low/no fluid, bad transmission. Need a lot more information. Year, mileage, maintenance history, etc. Remember that you are the eyes and ears of your helpers. So, you need to be a good witness. Do provide as many details as you can about the problem.
When i put it in gear it grabs as if to move foward but when given gas it just reves theres no grinding sound or anything like that but when i was driving it before it went out i had to drop gears all the way down to the lowest gear before it finaly wouldnt go any more in any gear even reverse