Two options - though note that if it's done this spontaneously it's most likely option 2 that'll work.
Reset the Toolbar
1) Go to View -> Customize
2) Click the "Toolbars" tab
3) Select one of the "Layers Palette" toolbar options.
4) Hit the "Reset" button.
5) Choose the second "Layers Palette" toolbar and do the same.
You can also just hit "Reset all" but that depends on how much work you've done.
Option 2; Reset the workspace to factory defaults
Instructions found in the first post of this topic -
https://forum.corel.com/viewtopic.php?t ... 82b8faa3b7