Attack step & change of control

[TheShop]

My opponent swings with vanilla 4/4 beater.  After he declares the creature attacking, I cast reins of power.  The creature is untapped...can I now block with the creature(blocking himself????)?  Is the creature still attacking me? If creature does block, does he kill himself?

Similar shenanigans could occur with an active Magus of the Unseen + an opposing lodestone golem.
Thanks,

This is probably an easy question, I just have never seen a clear answer on it.

[Clariax]

When a creature changes control, it is removed from combat.  So a creature you gain control of can be declared as a blocker, but cannot block itself because it is no longer attacking.

[AmbivalentDuck]

During my opponent's declare attackers step, I gain control of Grizzly Bears using an instant.  May I use it to block during the declare blockers step?

Different situation: My opponent blocks my Grizzly Bears with their Hill Giant, I use an instant speed effect that gives me control of my Grizzly Bears.  Does it survive combat?

[Delha]

During my opponent's declare attackers step, I gain control of Grizzly Bears using an instant.  May I use it to block during the declare blockers step?

Unless you use some means to untap them, the bears will be unable to block due to being tapped.

Different situation: My opponent blocks my Grizzly Bears with their Hill Giant, I use an instant speed effect that gives me control of my Grizzly Bears.  Does it survive combat?

The bears will die. Gaining control of a creature you already control won't count as a change of ownership.

[Clariax]

During my opponent's declare attackers step, I gain control of Grizzly Bears using an instant.  May I use it to block during the declare blockers step?

Different situation: My opponent blocks my Grizzly Bears with their Hill Giant, I use an instant speed effect that gives me control of my Grizzly Bears.  Does it survive combat?

If you control the creature when declaring blockers, the creature can block as normal.  If something else prevents it from blocking (such as being tapped, though I'm not seeing any mention of the Grizzly bears being tapped in the question), it won't be able to, of course.

Changing control means it goes from one controller to another.  If the same person controls it continuously, then control never changes.  Also note this has nothing to do with who the owner is.  "Owner" and "controller" are two different things, though often are the same person.  The controller of an object can change, the owner cannot (excepting ante)

[Delha]

If you control the creature when declaring blockers, the creature can block as normal.  If something else prevents it from blocking (such as being tapped, though I'm not seeing any mention of the Grizzly bears being tapped in the question), it won't be able to, of course.

My apologies to AmbivalentDuck on this. I believe I had the mental notion that the bears were attacking in both scenarios (probably from remembering the second half of the post, and from noting the "declare attackers" step).