First of all, don't panic! Sometimes we think things are lost when they've been there all along!
To locate your assignment, do the following:
Refresh your page.
Make sure that your teacher assigned it via Google Classroom. Only assignments posted in Google Classroom will appear in Today.
Check your upcoming and done lists. Maybe you accidentally put a start date later than expected or you marked it as done.
Verify your internet connection. If you're not connected to the internet, new assignments posted by teachers will not show up.