photoshop erase

Set History options to Non-linear History in the History pallet, and history states to 50 in preferences.  If you don't go over 50 moves on an image you can back up in history to any previous state.  If you want to get back to the original image any time just go to the top of the history pallet and click that state.  You are now back to when you opened the file.  

If you didn't close the image, you can use the History panel. Click the snapshot at the top of the panel.

Again, if you haven't closed the image, you can selectively revert an image by selecting a history state in the History panel and painting over the area with the History Brush in the Tools panel.

If the file is still open in the session that you erased it, File>Revert (F12) should bring it back.

Unless you closed it out, there's also the File > Revert command to get you back to the original. If you did not think to set up Time Machine, hopefully you have a copy of the original somewhere.

Could you please post screenshots with the pertinent Panels (Toolbar, Layers, History, Options Bar, …) visible? 

If you have saved it and closed it I'm afraid this is an hard work.

Maybe you have a change if you have activated time machine on your Mac.

Otherwise there is the photoshop history mechanism: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjXof3XnvPwAhXz5OAKHfNZDdoQFjADegQIAhAD&url=https%3A%2F%2Fhelpx.adobe.com%2Fphotoshop%2Fusing%2Fundo-history.html&usg=AOvVaw1AbHlwFiYXhERJ3cMppJOO

If you have used a destructive editing technique/tool like the Eraser Tool then those pixels are gone. That is, unless, you only recently made the edit and you can still use the Undo command.

Not to be that guy but…

In the future try and always hide pixels using non-destructive techniques like masks. That way you can easily reveal pixels if you need to at any future stage.