Come over is a phrasal verb, that can mean several things.
To move from one place to another, or move towards someone.
For example: "Come over here."
To seem to be a particular type of person.
For example: "Politicians often come over as arrogant."
To be influenced suddenly and unexpectedly by a strange feeling.
For example: "Don't stand up too quickly or you may come over dizzy."
Overcome is a verb, which means to defeat or succeed in controlling or dealing with something.
For example: "Using technology can help many people overcome any disabilities they might have."