Login In Webpage Not Found

Yesterday the homepage was changed from the original one, to a new page. It was done through Settings > Reading > and selecting the new homepage. It’s now been brought to my attention that the link that was originally set up for logging into the backend (link provided) no longer works, and keeps showing an “Opps! This page can’t be found” error, so now no one can go into the backend and log in to access the dashboard.

Does anyone know what the cause of this error is and how to fix it? Any help would be appreciated, thanks!

The page I need help with: [log in to see the link]