Blizzard says we shouldn't worry though because achievements aren't lost. Servers are "really busy" handling new players right now, and achievements are "being hammered" particularly.
All we've unlocked is still there, say the studio. Our unlocks haven't been deleted but it's taking so long for the data to be called from the servers that they just appear that way.
"As you can probably imagine, our servers are currently really busy handling all the new players coming into the game," said Blizzard. "The achievements server in particular is being hammered, which may be why your achievements are not showing up. It is not that they are deleted, but that there is a (very long) delay in retrieving that data."
"We are working on it."
Diablo III, despite offering full single player, requires an Internet connection where our characters are saved on Battle.net servers. It's launch day saw the servers buckle under the immense deluge of traffic as new players scrambled to activate their copies and play. Blizzard says it needs Battle.net for co-op and to combat cheating.