TL;DR
Filters change what you see without deleting tasks. Check class and task-state filters whenever work seems to disappear.
Find your work
Open Tasks to review work across dates.
Check the selected classes and include the class you need.
Check task states: completed or deleted work will not appear in a view limited to unfinished tasks.
Open a matching task to verify its deadline and next work session.
Switch to the calendar view that suits your question: a week for your schedule, or a wider view for upcoming deadlines.
The date shown in a calendar may be the due date or a planned session. A task can therefore be absent from today without being missing.
After investigating, choose the filters useful for everyday planning. See restoring or reopening work and planning tasks.