I changed something in the functions.php file and now entire site doesn't work

try and use FTP https://codex.www.ads-software.com/FTP_Clients to replace the broken functions.php with an unedited version from your theme.

How do I know what my ftp address is?

if you don’t know, contact your hosting provider for support.

if you don’t know your FTP access data, you might not have set that up yet.

contact your hosting provider for support.

or try to work with cpanel if this is part of your hosting…