Fatal error message – please help!

Yesterday I was exploring themes in wordpress, liked one of them and clicked “activate” by mistake. When I typed in my URL https://www.messagestolove.com the following message came up: Fatal error: Call to undefined function wp_get_theme() in /nfs/c07/h04/mnt/113298/domains/messagestolove.com/html/wp-content/themes/gommero/functions/theme-options.php on line 9 When I try to log in using my usual login link messagestolove.com/wp-admin/ the same “fatal error” comes up. Someone suggested another admin page to log into and it wouldn’t take my UN or PW. I need very basic instructions.
I’m at a very basic level with wordpress so if you could speak to me like I’m a little kid it would be appreciated. lol. My web designer set up my site about 2 years ago and I’ve been making small changes ever since, learning as I go along. I don’t even know the version of WP I have installed because I can’t get into wordpress right now to find out, I’m sure I’m due for an upgrade as I haven’t upgraded it in about 2 years. THANKS in advance!

Stefanie