I pressed SHIFT+DEL on source code file name in project tree in Visual Studio
and it became grayed. What does it do? How can I undo?
UPDATE: Refactored hot-key name in the title. My apologies to all the parties affected.
It's "cut". Generally, in Windows, it's the same as CTRL+X. You can undo it by either ignoring it, or pasting it back.
SHIFT+INS is the corresponding alternative to CTRL+V. Back in the DOS days, these were the shortcuts for copying, cutting, and pasting, I believe.
(And to be complete, CTRL+INS is copy)