Website down and can’t login

Hello @rahulusfbcm

Download your theme(https://www.themezy.com/free-wordpress-themes/203-interstellar-free-responsive-wordpress-theme) and Login to your site via FTP or cPanel and delete the old theme(interstellar) and then upload the new theme.
Make sure you make a backup of the old theme.

Thanks.

Thanks @wbcomdesigns

I deleted and reuploaded and get the same error. I reached out to the creator but in the meantime how can I at least get into my wordpress backend?

There are more than 1 default clauses . Better wait for the theme developer to correct the issue.
To check manual go here: https://php.net/manual/ro/control-structures.switch.php and check for changelog: Multiple default cases will raise a E_COMPILE_ERROR error.