Are you sure the assembly is updated (rebuilt) with no error before you open the 2D drawing?
the section view is not updated because the part ref. is missing?
So you used a section view, the line drawn was likely originally selecting on a part of geometry associated to the original (replaced) file. This new file doesn't have the same geometry. Edit the section line sketch, and break/delete/replace the constraints to the line associated to the new geometry.
Go to File > Find References and make sure referenced files are correct. Right click on view if see you can open the referenced component by clicking on open icon.
If yes then do force rebuild Ctrl + Q
If that doesn't solve, then right click on view and select show.
Surprise surprise... Deepak got it. Right click and selected 'show' and it was fine. Well, other than the dimensions aren't linked but they're there and easy to recreate.