There have been a few threads about this. It seems to be a bug where the Dock hides although it is set not to.
This appears to be related to waking from sleep.
The quickest fix is to press Command-Option-D. This is the shortcut to toggle Dock autohide.
Pressing it twice to hide and then unhide restores it (until it disappears).
While it should be fixed, please do consider a friendly suggestion to keep Dock hidden. It is still there if you need it, and not always staring in your face and using screen space as you are writing, reading or viewing video, for example. My Dock is always hidden, and never missed :-)