Scratch

Were you opening a really complex project, or maybe one with a really large file size? That can sometimes happen...

Did it say space was low, or that it was out of memory? Sometimes the two can get confused with eachother; and even though they're not all that different, they may not be exactly the same thing, either.

For now, try closing all programs that you don't need running other than Scratch; which may prevent it from ever happening again. Even further still, if it happens again, this post may inform you on how to keep it from continuing to happen.

Considering it only happened once, I don't think it's too much of an issue. If it continues to happen, come back here and I'll try to help you some more