I cannot remove the flat mixing blade, after mixing cake. It has never happened before, just doesn't move up down or sideways! it seems completely locked.

The blade sounds like it's jammed somehow perhaps simply with some food matter.
Ensure the mixer is not plugged in then try removing the bowl, locking the mixer in the down position and pushing up hard on the blade to try and dislodge it. If it comes free and lifts up then try wiggling and rotating it normally off the mount.
Sometimes with use the attachments can become slightly warped requiring more force to lift them before rotating off the mount.
I once used my own Kitchen Aid mixer with the dough hook attachment to make a whole wheat bread dough. When I was done the hook was a bit bent from the hearty dough. There was also some flour in the mount causing it to jam. Removing the hook attachment proved difficult but not imposable.
Good luck.