Car has reverse, but drive does not work

My car will go in all gears except drive. low drive first works. Recently if I let the car warm up drive gear worked fine, but not today. I know I need to take to mechanic, but wanted kind of a heads up of what issue maybe. I am a female that does not want to be ripped off. Meaning that possibly I will not need a new transmission. Thank you, i appreciate your assistance.