404 Error – my website no longer works, please help!

I made the mistake of changing the Home & Site URL (https://codex.www.ads-software.com/Changing_The_Site_URL) in attempt to switch which domain went to my WordPress website. Unfortunately I can’t undo this error because now it will not allow me back into WordPress what-so-ever to fix it. I have no idea what to do. I either need to get it to work with the new domain or just get it back to the old domain for now… really whatever gets me back into WordPress so I can continue to build my website would be fantastic at this moment.

Please help me. Thank yo in advance.