I could not find any information on what would happen if you left it in too long. But I recall hearing something about them becoming embedded in the skin/ wall if left in too long. I think they can also cause scarring and sterility if left in. (I could have this confused with the old IUD from the seventies- not sure.) If you have to beg, borrow or steal to get the money, you should make an appointment to have it removed as soon as possible. Please DO NOT try to remove it yourself.
If you are in the United States, you can call around to your local health department and see how much it would cost. I know you can get medical procedures done there for much cheaper than you would pay at the doctors office. You could also call your local Planned Parenthood office to see if you can get treatment there.