1. The reason for the issue can be your installed plugins, please deactivate them one by one and you will be able to catch which one was the source of the issue.
2. If you want to not lose all the changes made with the plugins, please install 3rd party plugin staging, clone your website database and make the changes on the cloned staging site, here is the video guide to better understand how it works.https://www.youtube.com/watch?v=YtY-q5BBpeE
3. If the instructions above cant help, In order to assist we need to check your website from the back end, please install this 3rd party plugin “Temporary Login Without Password Plugin” which allows us to access your dashboard without sharing access details. To better understand how the plugin works, please watch the video guide below: https://www.youtube.com/watch?v=EMu0e78OpJo
Please make sure to mark your reply as private to hide it from the public.