The visual editor customization buttons are shown in a single row by default. Clicking on the last button will display another row. In a few occasions, these buttons disappear when your WordPress update attempt fail due to various reasons.
To fix the issue, you'll have to back-up your site and then re-install your WordPress installation under Dashboard >> Updates >> Re-install Now.
If you not comfortable with reinstalling your WordPress at the dashboard, please reach out to your web host to reinstall WordPress at the backed.