check the fluid level, does it work in reverse? also. make sure the shift cable is connected to the trans and that it is properly adjusted. if it is, drop the pan and check for metal shavings in the pan and make sure the filter is properly secured, make sure the manual valve is properly engaged into the valve body. it will not move forward in OD, D, 2 or 1?
i love how people chime in and say the trans is shot (which it may be) without any troubleshooting