wont shift into any gear, unless i put it in 4wd low? any ideas?

i drove home from work with no problems, and when i got back in to go to the store it wouldn't shift at all unless i put it in 4wd low. this is the only vehicle my family has and we have very little money, any sugestions would help.but if you don't have a clue please don't waste my time or money. thankyou for your help.