This page is relevant for Mac users only.
From the desktop go to the little Circle with an Arrow on it along the Navigation Bar at the top. That is the Backup icon.
If the Icon has an Exclamation Point(!) inside the circle, that means something is wrong. It should look like this.
Click on the symbol. And then click on Open Time Machine Preferences.
This will show you if there are any errors on the backup. As well as show you the last time your machine backed up to Time Machine and the oldest back up.