Hi
That trick with the plastic screw should work, it's the manual release for the door but it's possible that the connecting part which unlocks the door may have become dislodged.
Perhaps when it's next open you could check out the locking mechanism and see that nothing looks ceased and if so give it a dose of WD40 and cycle it. Sorry, that's all I can suggest so otherwise you'll just have to give Dyson or your local appliance service centre a call.
It must be pretty annoying. Hope you get it working again.