Will not shift out of first gear

I just bought the car and had to replace the intake gasket. After I fixed that I drove it for about an hour, and it hesitated slightly shifting from first to second, and then after about an hour of driving it, it I went to take off from a stoplight and it would not shift out of first gear. It goes into first and reverse fine, but no matter if I try to manually shift it or not it will not leave first gear.