Hi there! I'm sorry to hear that you're having trouble with the 'Tab Color' menu item in Excel. Here are a few things you can try to enable it again:

1. Check if the worksheet is protected: If the worksheet is protected, you won't be able to change the tab color. To check if the worksheet is protected, go to the 'Review' tab and see if the 'Unprotect Sheet' button is enabled. If it is, click on it and enter the password (if there is one) to unprotect the sheet.
   
2. Check if the workbook is shared: If the workbook is shared, you won't be able to change the tab color. To check if the workbook is shared, go to the 'Review' tab and see if the 'Share Workbook' button is enabled. If it is, click on it and uncheck the 'Allow changes by more than one user at the same time' option.
   
3. Check if the workbook is in Compatibility Mode: If the workbook is in Compatibility Mode, some features may be disabled, including the 'Tab Color' menu item. To check if the workbook is in Compatibility Mode, go to the 'File' tab and see if the 'Compatibility Mode' message is displayed. If it is, click on it and select 'Convert' to convert the workbook to the latest file format.
   
4. Check if the worksheet is selected: If the worksheet is not selected, you won't be able to change the tab color. To select the worksheet, click on the tab name once to activate it.
   
5. Check if the worksheet is minimized: If the worksheet is minimized, you won't be able to change the tab color. To maximize the worksheet, click on the 'Restore Down' button (the middle button between the minimize and close buttons) on the top right corner of the Excel window.

I hope one of these solutions works for you!