Key will not turn in the ignition to start car. Any suggestions?

I don't know what nationnal you are living . This is a simple issue in Vietnam . This damage was caused by the breaking of small marble (ball) and small spring be inside ignition lock cylinder . First you conduct as Tom's advice , or fill into it some oil . If not , you should disassemble detail the ignition lock cylinder and replace the damaged parts . If not either , finally , must replace the new