why wont my 2004 chevy impala stay running

The mechanic messed up?

whose idea was it to change the injector? if you have a mechanic, how long have you known him ( mine just tried to manipulate me over throttle body). The first thing I would consider is the fuel relay in the fuse box ( because you said the car had already been started and was running). Secondly, I would check for looseness of the battery cables. If they are loose/ off then it wont run either. good luck

Also check the fuel lines for leaks and check for a disconnected vacuum hose for the EVAP system or the fuel pressure regulator. Hope that helps! Jim