While teaking around my mesh in editmode, I inadvertely activated an unknown function. The lower arm of my meshbody can not be selected or edited anymore.

Objectmode: Full body is show, inclusive lower arm
Editmode:
* Solid Display: Lower arm is nomore visible
* Wireframe Display: lower arm is visible, but dimmed (can't select any part of it)

What's the name of that function?
How can I turn it on/off?

Thanks for adivce.

found it - vertices where hidden
Editmode: Mesh > hide selected (HKEY) hides all selected verts

Made them visible again with ALT HKEY.