Dock not showing when mouse touches bottom of screen

I have installed nothing new, no upgrade either from yesterday to today. I have just turned my iMac off yesterday and turned it on today. But today my dock is not showing when the mouse touches the bottom of screen.

I have autohide ON on system preferences and dock is adjusted to show in the bottom.

Dock shows/hides when I press Cmd-option-D. The problem is just with the auto hide/show.

Yes, I have restarted the computer. I have tried also to restart the finder and the dock several times.

Yes, I have also deleted ~/Library/com.apple.dock.plist
trying to force the preferences to rebuild.

Nothing.

Any clues?

thanks in advance.