The Tips in detail

 "I can't open a previously saved scene."

There are some reasons that might be the cause for this happening. Perhaps your HDD made an error while writing or you suffered from an AC-failure (power surge) or the scene is way too complex for your machine (which means you are out of memory in most cases). Perhaps you need a certain plugin neccessary to reconstruct the scene. Anyways, try merging the file. In many cases this works quite well. If this doesn't work, try opening (and / or merging) the file on another machine.

Sometimes Max chokes on very complex objects, which might lead to models you can't load. Try a version of this model one file older than the one you saved and failed to load (provided you have turned on increment on save and an earlier version at hand). If your previous file is dramatically outdated (ie. to many changes between last saved versions) try your backup (autosave) instead. Don't forget to rename the .mx to .max before trying to load ;)