...later, after starting the quest, I went back in to Geirmund's Hall and the amulet & boss weren't there, though the quest arrow - marking the amulet - was there hovering over the water. I searched everywhere. No amulet fragment.

What do I have to do to get the dungeon to reset?

I'm on a PS3.

Thanks!

Have you tried waiting 30 days? Dungeons are usually set after 30 in-game days. Not sure if this will fix, but it's worth a try.

Not formally. I went back in it after initially clearing it and battled some NPC, but I don't know how much time had passed since I'd cleared it. I'll try and wait 30 days for sure and see what happens.

Btw, Does anyone from Bethesda every check these threads? Or does this need to be posted elsewhere?