NOTES text disappears
It's a bug in PowerPoint that seems to affect PowerPoint 97 and PowerPoint 98/Mac users. It appears to have been corrected in later versions.
To keep it from biting you again, choose Tools, Options (or Tools, Preferences on the Mac), click the Save tab and remove the checkmark next to Allow Fast Saves.
That should prevent this problem from recurring, though it won't bring back your lost notes text.
Next, you'll want to get at least the SR-1 patch and possibly SR-2 from Microsoft to fix this and several other PowerPoint 97 and Office 97 bugs.
Even after installing the patch, try not to save your presentation while you're in Notes view. This seems to be what triggers the bug in the first place.
If you've lost a huge amount of notes text, it may be worth trying to retrieve it by saving a copy of your presentation then editing the copy in a text or binary editor. You may be able to locate and copy the text from it into your notes pages in PowerPoint.