Please see if any of these help:
1. You may want to check the file extension to be sure that it's an Excel file.
2. See if closing the Excel file before trying to connect to it in Tableau helps.
3. If that doesn't work, try to refresh the data source and check.
4. If that too doesn't work, try to edit the connection and verify that it's connected to the right file.
5. If it still doesn't work, try making a copy of the original Excel file and see if you can connect to the copy and find all the sheets available now.
6. If nothing works, try restarting Tableau and check.
7. If it still doesn't work, give it a break for some time and try later. Your system is probably too tired with all the hard-work you have been doing.
Save the excel file on local path.
Delete the data source from Tableau.
Close and then, re-launch Tableau
Add new datasource and select the excel file