Hi Melanie,
We apologize for this inconvenience, we’ve identified the underlying cause of the problem as a bug that occurred with the rollout of our new Full Screen Editing feature in the last Post and Page Builder update.
This issue occurs when several buttons are added to the MCE controls. We’re tracking the bug in this GitHub issue.
There are a couple of ways you can troubleshoot right now. You can use a plugin like WP Rollback to revert Post and Page Builder back to the previous version, or temporarily disable some plugins that are adding buttons into MCE. Common ones are Jetpack’s “Contact Form” module, which adds a form button, and shortcode plugins.
We’ll be releasing an update to fix this as soon as possible.