Can't get to my admin log in page!

Hello all,
first of all I should explain that I’m a little thick when it comes to technical problems but I’m trying to learn and that’s how this problem occured!
I have a site up and running and today I learned some new SEO tips so started to implement them.
One of them was to put the keyword in the page URL and I saw the message from WordPress SEO plugin that I didn’t have this on my homepage, so I went to settings> general> WordPress Address URL and changed it from https://sheringhammarket.co.uk to https://sheringhammarket.co.uk/sheringham-market
I then saved changes and tried to view the page….. but an error 404 message appeared.
I realised that I’d made a mistake (again) and went back and changed things back to how they originally were, saved changes and tried to view the page, after refreshing the page I still get the error 404 message with just the background image displaying.
When entering the site URL in a new page I can still view the whole site, no problem, but I now can’t get to the admin to work on the site!

When I type in sheringhammarket.co.uk/wp-admin into the address bar it automatically changes to sheringhammarket.co.uk/sheringham-market/wp-admin and leads me to the error 404 page!

Is there a way to get back to my admin login page? and do I need to change the settings again?

To be honest…. and trying to think logically, when I changed things back to their original state and hit “save changes” I would have thought that this should have worked but as I’m finding out lately, WordPress sometimes seems to have a mind of it’s own!
Up untill a couple of weeks ago I had a button to enter H1 H2 H3 tags etc. but now it’s gone!

Any help that anyone could offer would be greatly appreciated.
Thanks in anticipation.
Regards, Gary.