2001 chevy 4x4

can anyone tell me whats wrong with my truck ,its a 2001 chevy 4x4 it says service 4x4 and it wont go into 4 wheels drive anyone ever had this problem? thanks

get the dealer to scan it. could be bad module, switch, encoder motor, sensors or damaged wiring.
gm has a problem with the encoder motors and has updated parts i don't know if your truck applies to the list but worth a check.