Hi,
I modified the sign up/ log in pop up and now it is not working.
I tried to delete it and insert it again but it does not works (it keeps appearing the same).
Is there any way that I can restore this element so that it appears as it was originally?
Please help! 

I think you might be using Bubble’s default sign-up popup.
You can save time by creating a new bubble app in your account, and copy/paste that popup with workflows into your current app.
Delete the unused app unless you want to use it as a sandbox … I actually have a sandbox app for experimenting and learning… 
It worked now! Thank you, you are very kind! @cmarchan 
Do you know any way that I can create just a log in pop up, without sign up? And create the users and passwords manually from the editor’s database? @cmarchan
Show and hide the groups selectively
How can I create a password for a user? Where are passwords saved? Because I don’t see them in the “user” data type @cmarchan
Bubble does not show user passwords due to privacy. This is by design.
There are ways to create and manage passwords yourself though. It is somewhat intricate to explain. I remember a video from Coaching No Code Apps on this. Perhaps you can look it up.