Can a computer be taken over by another source?

This question is a broad questions and can be answered in many different ways.

Yes.

With today's technology you can even buy software programs that allow you to gain access to your own computer remotely. Let's say you have files on your home computer and you need something from it while your at work, this is one way it could happen.

If you allow a virus or other harmful software to run on your computer it may do things others want it to do with out your permission.