A takeover: another website pops up instead of my website!

When I try to login to https://www.adafagan.co.za admin I got: https://www.wakkerstroom.co.za/login/?redirect_to=http%3A%2F%2Fwww.adafagan.co.za%2Fwp-admin%2Findex.php&reauth=1
This problem of Wakkerstoom site taking over, happens most of the time.

And when my icon does show up, I think my site is working, but I can’t login into admin backend with my username or password. And when request a new password, the emails I receive for new password, their link doesn’t work. Just tells you to ask for new password again, again. Can’t get get any help there!

And if you act as someone searching the internet for Ada Fagan’s website, you get: https://www.adafagan.co.za/artist-style-of-ada-fagan/ The Wakkerstroom’s site pops up there too!

And if my website does show up when searching the internet, My site doesn’t work properly. You click the menu to get only one page, it doesn’t show the other drop-down pages, either.

My internet provider can’t help me, and say it’s a WordPress problem.

How come another website is linked to mine?
And how can it be fixed?

WordPress please help sort out the mess.

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