Will a copyrighted source code, closed or open source, will be in public domain once its copyright expires?
In Copyright systems which serve the Public Domain, yes. All works, upon expiration of Copyright, are released into the Public Domain that year. There are some countries which do not have Public Domain within their Copyright regime, but I'm not aware of which countries these are.
But any country that is a signatory of the Berne Convention has a minimum Copyright term of 75 or author's life plus 50 years. In the United States, since the fairly recent Copyright extension act, that term is 95 or author's life plus 70 years.
For closed source, can I ask the company for the closed source code once it got expired (just assuming based on what I've understood so far :-) )?
If they don't release it, they are under no obligation to provide it. And, since most countries also have Trade Secret protections, their source code is protected as strongly as if it were under Copyright, until it's disseminated.
Assuming question 1 is true, how can I prevent a source code from getting into public domain once the copyright expires, if it's possible?
Under most systems, it's possible to re-introduce a work under Copyright if the new version has been sufficiently "transformed." The specific requirements differ between nations.
I'd like to point out, though, that re-introducing a PD work under a new Copyright has an unfortunate tendency to reduce the market for the work. There are numerous examples of this and numerous studies. Boldrin and Levine explain in detail.