Here are a few things you can try when your sheets or workbooks are not opening in the same instance of Excel:

1. First, try closing Excel, then reopening and check the behavior.
2. Restart your computer and try again.
3. Check to make sure you're using the correct version of Excel for your file.
4. Check to see if a system update is required.
5. If Microsoft Excel is installed as part of Microsoft Office 365, make sure the subscription is active and up-to-date.
6. Try repairing Microsoft Office using the Microsoft Program Install and Uninstall utility.
7. Run a full system scan to identify and resolve any system issues.
8. Check to see if your anti-virus software is blocking Excel from running.