If you open an attachment from an email, the first thing you should do before making any changes is Save As..., saving the document to your N:drive (Documents), or to a suitable location on your computer or to a USB Memory Stick. Once you have done this you will be be able to continue working on the document as normal, secure in the knowledge that any changes you make (and subsequently save) will be retained.
When you open an attachment directly from an email, the email program (whether a "client" or a webmail interface) saves a copy in a temporary area on the computer's hard drive (C: drive). The location of this area varies depending upon the settings on the machine but it is usually within a folder with the name "temp". Documents can sometimes be found before logout using by searching the C:drive.
If the worst happens however, and you forget to Save As, you can try to find your document as follows:

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This is question number 1648, which appears in following categories: