what happened to the king?

Hrothgar was killed by Grendel's mother off-screen and she took his place (being a shapeshifter) to make Beowulf a king in no time, as she promised. There is no logical explanation why he would kill himself after his "curse is lifted". Also, notice the golden trail under the water when the ocean waves cover Hrothgar's body.

I'm of the opinion that he threw himself from the balcony now that the burden of the curse was lifted. He knew the deal, having left her alive--Beowulf had now been bestowed with the curse which came with the crown as well. The only way to pass a crown is with death.

The king probably also knew of Beowulf conceiving another child with the demon since he had been coerced into doing the same.

The gold flash seen in the water was Grendel's mother who had come to claim the kings body knowing he would step aside.

Hrothgar committed suicide because, A, while his curse was lifted, his queen, the woman he did infact love, wouldn't give him a child B, knew the curse was past onto Beowulf the moment the two spoke in private(the sight of the demon, the fact he claimed that he wouldn't have escape if he hadn't had killed her, etc)