Check:
1. Availability — are there time blocks for the current + upcoming days?
2. Buffers — are they so large that no time remains?
3. External calendars — is your work calendar fully booked?
4. Min. notice — are you looking at time slots beyond the "Min. notice"? Slots within that period are not shown.
5. Override — did you accidentally set a "not available" override for today?
Usually it is point 3 or 4.