wont turn off


hi guys i have a 1986 2.8 non turbo rocky and i just bough it a few days ago, its a little beat up by time but its my project car. now the thing is that engine runs great but i cant turn it off with the key i have to turn it off manualy by pulling on a string that kills the engine. now i would like to know if it is something serious or is it just a matter of buying a part and it will work just fine.