This is a beginner topic. Little to no advanced knowledge is required.
Level 4 or Higher - See permission level document for details
If you made any critical errors, or deleted content, or just broke your site in general, this page will help you create restore a previous backup of your live site. Keep in mind that any changes made to your site since the date of your last backup WILL BE LOST.
Table of contents
The dashboard is located at https://dashboard.pantheon.io.
Select the sandbox that contains your live site. See below.
First click to open the Live server environment. Don't worry you can't break anything!
Next click "Backups"
Click the "Restore" button next to the backup you want to restore. Keep in mind that any changes made since the date and timestamp of this backup will be lost.
Now you can try your change again and see if you have more success.