There are a few things to check if you are not seeing the PyXLL ribbon tab.
1. You have a ribbon xml file configured
In your pyxll.cfg file you will see a ribbon setting. This should point to one or more ribbon xml files that can be customised to change the ribbon. If you are using the example ribbon that came with PyXLL then you will just have a single file "./examples/ribbon/ribbon.xml" configured. If you have removed the config setting or the ribbon file no longer exists then that will explain why you are not seeing the ribbon tabs.
If you're not familiar with how the ribbon works and is configured using PyXLL then please see Customizing the Ribbon in the PyXLL docs.
2. The PyXLL COM Add-in is not disabled
PyXLL automatically activates a COM component that is part of the main PyXLL add-in when it's loaded. It's this COM component that provides the ribbon customization in Excel. If this component has become disabled for any reason then PyXLL will not be able to activate it.
Check your Excel add-ins for any disabled add-ins by going to File -> Options -> Add-Ins and then select "Disabled Items" and Go it will show you any disabled ad-ins. If the PyXLL COM Add-in is listed in there then re-enable it.
3. The ribbon tab has been hidden
Another reason the PyXLL menu would not be showing is if it's been removed from the ribbon. You can check that by right clicking on some empty space on the ribbon and select "Customize the Ribbon...". There you will see the available tabs and if the PyXLL tab is not ticked then you can activate it by ticking it.
4. Check for other errors in the log file
If you have checked both of the above then enable debug logging by setting "verbosity = debug" in the LOG section of your pyxll.cfg file, restart Excel, and check the log file for errors. You should see the complete ribbon xml in the log file.
If you are not sure how to find the PyXLL log file then please see Where is the PyXLL log file located?