How many miles are on it?
Has the transmission fluid ever been serviced?
Did the check engine light ever come on either just before this problem started or on now when the engine is running?
Yes on service
No on engine light
the first thing you need to do is check the transmission fluid level on the dipstick for being low, and also check the color and smell. if extremely dark fluid or a burnt smell, then you need to change the fluid and see if the transmission engages after that. If the fluid is black and really burnt smell, there is a good chance of internal damage and even after the fluid change that it wont go into gear. But this is the first step you need to take, and then go from there.
If it doesnt go into gear after the fluid and filter change, it will need to be towed in and then inspected for internal damage(bad pump, bands worn out, etc)
make sure to use Mopar ATF+4 fluid