Can I call functions from a different sketch?
You can only use one sketch at a time, meaning one sketch can be stored inside an Arduino to be executed.

This means all your code (and functionality for all devices you want to access) needs to be within that single sketch.

So what you mostly need to do is:


Add a library for every device that you want to use.
Call functions from that library in your .ino file.
If you want to add functions which are not part of a library, you add both the function code and the call (where you need it) in your .ino file.


Also if you are new, don't start with everything at the same time.


First find an example how to control the display (that's probably easier than the CAN shield).
Add the library for the display
Change (or copy from an example) code for the display
Run and debug it until it works.
Create a new sketch for the CAN shield.
Add the library for the CAN shield.
Call functions from that library (or use an example if available).
Once you are convinced you know how to use both devices (display + CAN), combine the sketches (create a new sketch, add both libraries, copy code and make it one sketch).