Hello @sartoric, thanks for your answer.
I am 100% sure the 24/7 monitoring was enabled.
However, because I couldn’t find any information about this issue I did a factory reset (https://www.suunto.com/fr-fr/Assistance/faq-articles/lifecycle/comment-reinitialiser-votre-montre-suunto/) yesterday and it’s now working. So I guess it’s a solved problem…
The problem occured after letting my watch charged a whole night. I had disabled the 24/7 monitoring during the charge because it was useless. The morning, I put it back on my wrist and enabled the monitoring. Even after several hours nothing showed on the screen.