You show up at the customer site and realize that the project you have is completely different than what is in the PLC or there is no project backup at all. You can create an empty TwinCAT project and update your project from Target. 

You might be getting the warning below when you try to login which indicates there is no matching compile information.

 Click No and right click on the PLC project and select 'Update PLC from Target' 

Click okay to project is different warning. 

If some of the POUs have 'warning sign' next to them, try to 'Update PLC from Target' again.

Clean the solution and build the solution. 

Unload the project. 

Then reload the project.

You should have successfully updated your project from target after this step.