Hello!

If I lock my desktop and log in then again, the display will be black.
I can't change this black screen. But I can move the cursor.

How I can resolve this problem??

Thank you for answers!

Either the lock wasn't removed or there may be a a GL error (thus invalid textures) - latter esp. in case you're using an OpenGL screensaver on top of the lock.

-> Try to suspend the copositor by pressing "Shift+Alt+12"
If that doesn't restore visuals, you'll have to elaborate on configuration and conditions, ie.:

- what kind of screenlocker you're using (in doubt make a screenshot of the "kcmshell4 screensaver" dialog)
- whether you just (explicitly?) locked the screen or the screen was locked by inactivity or as result of eg. a suspend to ram

I update my system. Now it is working normally.

Thank you for answers!