To reopen a document you’ve already saved and named, you can pursue any of these avenues:
Open your Documents library (or whichever folder contains the saved file). Double-click the file’s icon.
If you’ve opened the document recently, choose its name from the taskbar’s jump list.
If you’re already in the program that created the document, choose File→Open. (Or check the bottom of the File menu, where many programs add a list of recently opened files.)
Type the document’s path and name into the Run dialog box (+R) or the address bar. (You can also browse for it.)
The Open dialog box looks almost identical to the Save As dialog box. Once again, you start out by perusing the contents of your Documents folder; once again, the dialog box otherwise behaves exactly like an Explorer window. For example, you can press Backspace to back out of a folder you’ve opened.
When you’ve finally located the file you want to open, double-click it or highlight it (from the keyboard, if you like), and then press Enter.
Most people don’t encounter the Open dialog box nearly as often as the Save As dialog box. That’s because Windows offers many more convenient ways to open a file (double-clicking its icon, choosing its name from the Start→Documents command, and so on), but only a single way to save a new file.