pontiac 2001 montana hit a rock under van . can shift gears would not go forward nothing is leaking can only drive in reverse hel.

Check the shift linkages. They attach to the trans at the top, rear. Down below the master cylinder area. It's possible that the linkage slipped due to the impact of the rock.
Check the transmission mount (viewable from driver's side through wheel well).
check the front engine motor mount (on the drivers' side at the bottom of the motor, behind the crankshaft pulley).
If the impact was strong enough it may has cocked the engine in such a way the the linkage can't travel as far as it needs to in order for you to be able to shift into drive.

Check the shift cable (runs from shifter on transmission, underneathe the van and then into the interior. The rock may have smashed the cable cover onto the cable keeping it from sliding insode of it's cover (the way it normally does).

Having given more thought to your problem. Check the transmission pan for a large dent. The transmission pan is located on the bottomof the transmission on the drivers' side of the van. It is somewhat "L" shaped. If the rock that you hit put a large enough dent in the pan (an in just the right place) it could prevent the internal shift linkage from moving forward (or backward) enough to engage the "drive", thus limiting you to park and reverse.

I this is the case, take the van to the shop and have the fluid drained, pan removed, dent repaired or pan replaced (while pan is off inspect linkage for daage and for proper operation). Replace the transmission filter and then reassemble for test drive.