This appeared on mine too yesterday. I would bet it's due to the new LibreOffice 5.2 update in their PPA.
Try these in a terminal:
sudo apt update
sudo apt dist-upgrade
sudo apt dist-upgrade
I believe it needs to run one set of updates first (e.g. LibreOffice), then run another set of updates (e.g. kernel), and then it should return back to normal.
Also, if this is caused by LibreOffice 5.2, you'll need one extra package to"fix" the theming: libreoffice-gtk2