Fatal error, I can't go to any page.

Sounds like you should reinstall the theme – you can first delete it by using FTP (or other file manager your host provides) to find it in the themes folder.

Sorry, I’m a total newbie. I don’t know what FTP is and where is the themes folder?

FTP is a way to access files on a remote server – it’s not part of WP. See:

https://codex.www.ads-software.com/FTP_Clients

You’ll need to ask your hosting company about how to set it up for your site.

Once that’s done, you can find the themes folder here:

wordpress
   wp-content
        themes
           customizr

Note that if you have changed any theme files, those changes will be lost when you replace the theme.

If you are concerned about that, it would be a good idea to back-up your site before doing this –

https://codex.www.ads-software.com/WordPress_Backups

Also, for future reference, you should never modify theme files – any changes like that should be made in a Child Theme –

https://codex.www.ads-software.com/Child_Themes

I’m using inmotion hosting. Do you know where I can find the themes folder? From the first link you gave me, it looks like I have to go to the control panel, right?

Oh I got it!!! Thank you so much!