https://www.chess.com/blog/CHESScom/chess-is-booming-and-our-servers-are-struggling
Activity on the site has doubled in recent weeks, and the servers are struggling to cope. They're scrambling to improve things, but it's not something that can be completely resolved immediately.
Just tried to check the results in a Multi Club Arena, which took place yesterday and had to take notice, that all results of completed Arenas disappeared.
Checked this in a few other clubs, from which I know, that they regularly organize Multi Club Arenas and found all of these lists empty.