Had same problem.
Clicked on little white flag ( action centre) as mentioned before, looked at and deleted the 'actions'. Problem solved.
This also worked for me - after wasting 40 minutes searching the internet for solutions. Microsoft - please do something so that people don't have to waste their time trying to figure this out. I don't think that most people would think to open up the little
arrow at the bottom of their screen and start clicking on random icons. I needed to get some work done on Excel, which is a complete impossibility if you need to use the horizontal scroll function. Not happy...
The real problem is when taskbar gets unhidden is because a program wants your attention, but, the icons in the notification area (in my case) won't blink or display any alert forcing me to click on every one of them in order to find out who wants it, then
the taskbar autohides again.
It should be easier if they'd blink or display an speech balloon but that comes from the software designer.
I ended up eliminating the useless action center (google it, it will show how to do it) it had me bugged off cuz it doesn't blink or attracts my attention in some way, can't Microsoft put the icons to bounce like Apple? now I have the same problem with dropbox
or other software every now and then