“Someone has changed my Windows 10 Lockscreen …”

and I can’t change it back.

Lockscreens are located in the C:\ProgramData\Microsoft\Windows\SystemData directory.

You’ll need to Take Ownership of that directory in order to delete the contents.

One way to do that is with a command line command:
Takeown /S "ComputerName" /u "domain\administrator" /P "password" /F \\"ComputerName"\c$\ProgramData\Microsoft\Windows\SystemData /A /R

Once you have done that, you should be able to browse into the SystemData directory, and delete the “”bad”” lockscreen.