Hello Sophia,
SNDS only reports on mail delivered to Microsoft's consumer services (Outlook/Hotmail/Live/MSN). It does not show data for Microsoft 365 work or school domains. If your last two days of sends were mainly to business tenants on Microsoft 365, "View Data" will be empty. Microsoft also updates SNDS once per day on a Pacific-time schedule, so yesterday's activity doesn't appear until after the nightly aggregation finishes.
Try to send a test from your Marketing Cloud route to a real outlook or hotmail mailbox. After it arrives, open the message's source and confirm the sending IP in the "Received from" line matches the IP you added to SNDS. Then check SNDS "View Data" the next morning once the Pacific-time job has run, in Madrid that's typically visible around 09:00-11:00 CEST. If that test message used a different egress IP than the one you registered, add the actual IP you see in the header to SNDS and use that going forward.
If you did send to consumer Outlook.com addresses and still see nothing the next day, reply here with the exact IP and I'll help you pinpoint whether Marketing Cloud used a different IP for Microsoft destinations or whether you're hitting a temporary SNDS delay.