Is the vehicle security light lit ? It could be a fuse or the starter relay , or even the ignition switch , or even the PNP - park neutral position switch . This is a complex electronic starting system , computer controlled . The ignition switch is a low voltage input to the the BCM - body control module . When the BCM receives this input to crank the engine a lot of complex commands are sent over the serial data network to the PCM to enegrize the starter relay , there by sending battery voltage to the S terminal on the starter solenoid . If your mechanic disconnected the battery it could have caused the radio to say locked , you need a code to enter into the radio to unlock it , may need to have the dealer hook up their factory scan tool to do that .
Your best bet , find a better automotive technician , not a mechanic.
Circuit Description (Key Start)
For ignition switch power modes refer to Body Control System Description and Operation in Computer/Integrating Systems. Once the ignition is placed in the Run/Crank position, the control circuit of the Run/Crank relay is grounded by the body control module (BCM). With the Run/Crank relay switch closed, battery positive voltage flows through it and on to the park/neutral position (PNP) switch. With the PNP switch in either the Park or Neutral position, battery positive voltage will flow to the starter relay coil supply voltage input terminal of the powertrain control module and the coil side of the starter relay. Placing the ignition in the START position sends a message to the powertrain control module (PCM) requesting engine start. If the PCM has determined that the transmission is in Park or Neutral and theft is not active, it will ground the control circuit of the starter relay. Battery positive voltage will then flow through the switch side of the starter relay to the S terminal of the starter solenoid, cranking the engine.