It falls to the ground as normal
Hover keyword does not affect falling prone. As per the compendium entry for
Hover: If a creature can hover, it can remain in the air if it is stunned.
Note that the compendium entry for
Stunned specifically confirms this:
Stunned: The creature falls if it is flying, unless it can hover.
Otherwise, it follows all the rules as normal for
Falling Prone: If a creature is knocked prone while it is flying, it falls. This means a flying creature falls when it becomes unconscious or is subject to any other effect that knocks it prone. The creature isn’t actually prone until it lands and takes falling damage.
Note that the
Stunned condition does not inherently cause a creature to be knocked prone, thus the special rule for flying creatures falling under the definition of Stunned, and the special counter to that rule in Hover.