Theme crashed after changing functions.php file, now can't access to undo

I changed the functions.php file (right at the top) to add a new function, obviously didnt work and now I can’t access ANYTHING. Please help me.

I changed part of the functions.php file to add something, and now nothing works. I tried to look for the original functions.php and upload it through my hoting account (webspace browser) but nothing changed.

How can I reactivate my old theme (the synergy carbon theme) and uninstall the corrupted theme (Gridiculous)? PLEASE help!! Thank you!!!