Hi, I think I can answer that. They realised that the building (and some others in the neighbourhood) was in grave danger of collapsing and had to immediately close down the Migros and evacuate the people who lived above it. I don't know when it's going to reopen.
Here's a link : https://www.migros.ch/fr/cooperatives/migros-geneve/actualites/40-fermeture-de-migros-servette-pour-raisons-techniques.html There were also some articles in the newspapers.