Change the switch after losing the key and now it will not start

I believe you have a "passlock" key. The new key has a different code in it than the original key. You need to retrain the vehicle to use the new key.

This is the process -
Fully Charge Battery
Clear all Diagnostic Trouble Codes (DTC). You can clear the codes by disconnecting the battery for ten minutes.

Relearn Procedure

1-Turn on the ignition, with the engine off.
2-Attempt to start the engine, then release the key to on (vehicle will not start).
3-Observe the security light, after approximately ten minutes the light will turn off.
4-Turn off the ignition, and wait five seconds.

Repeat steps one through four two more times for a total of three cycles/thirty minutes (The vehicle is now ready to relearn the Passlock Sensor Data Code on the next ignition switch transition from off to crank)

You have to have a mechanic program the ignition switch to the security system or it will never start.