See Golan v. Holder 565 U. S. ____ (2012).
The question was whether Congress could pass a law that caused works in the public domain to regain copyright.
The Supreme Court held that:
The text of the Copyright Clause does not exclude application of copyright protection to works in the public domain.
Historical practice corroborates the Court’s reading of the Copyright Clause to permit the protection of previously unprotected works.
So, yes, works can come out of public domain and into Copyright protection.
What about people who had been relying on the works not having copyright (i.e. people who had created derivative works, etc.)? That depends on how Congress intends the hypothetical law to apply to them. In the law in question in Golan v. Holder, Congress planned ahead for this, and included provisions for it in the law (quoting from Golan v. Holder):
Reliance parties may continue to exploit a restored work until the owner of the restored copyright gives notice of intent to enforce—either by filing with the U. S. Copyright Office within two years of restoration, or by actually notifying the reliance party. After that, reliance parties may continue to exploit existing copies for a grace period of one year. Finally, anyone who, before the URAA’s enactment, created a “derivative work” based on a restored work may indefinitely exploit the derivation upon payment to the copyright holder of “reasonable compensation,” to be set by a district judge if the parties cannot agree.
Last, could Congress extend copyright for 10,000 years? In principle, the Supreme Court has not said anything against this, but they have indicated that there is some line beyond which a copyright term would no longer be considered a "limited time" as required by the Constitution.
In Golan v. Holder they mention in passing hypothetical successive re-applications of Copyright to a work after it expires, calling it "legislative misbehavior":
the hypothetical legislative misbehavior petitioners posit is far afield from the case before us.
And in Eldred v. Ashcroft, they mention in passing hypothetical unlimited successive extensions of copyright, each of limited time, again calling that "legislative misbehavior":
Concerning petitioners' assertion that Congress could evade the limitation on its authority by stringing together an unlimited number of "limited Times," the court stated that such legislative misbehavior clearly was not before it.
I could imagine a court holding that the "limited times" clause in the Constitution does not permit Congress to extend copyright for 10,000 years, which is longer than the existence of the U.S. or any countries or even civilizations that the founders were aware of at the time of the Constitution's writing.