Some processes on your computer, such as security features in Windows or regular Microsoft Outlook updates, can disable add-ins that you installed in Outlook, such as the Marketo Sales Insight add-in. This would cause the Marketo buttons to disappear from your Outlook.
To re-enable the add-in, follow these steps:
Go to File > Manage Add-ins.
Locate the add-in "Marketo Insights" and check the box to turn the feature back on.
If the add-in is not listed there, uninstall the add-in using the Microsoft uninstaller, then reinstall the add-in.