Shortkey to open current explorer window again

Update based upon comments by OP

You can do this with the mouse. Right click on the directory and select Open in new window.

Or, for a short cut with keyboard, CTRL + N

Original answer below

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Your question is not clear.

If you want to navigate to an already open Window, then use ALT + TAB

However, if you want a short cut to any specific directory, then I don't think you can but there is possibly a work around to this (depending on your situation it may or may not be feasible).

You can assign global commands to shortcuts. So, create a shortcut to My Documents, and save it to your desktop. Right click on this short cut and select properties. Under the shortcut tab you can type in a letter in the Shortcut key box

For example, I typed G and it shows

Now, I just type CTRL+ALT+G and that directory loads up.

In Windows 7, simple click: Ctrl + N