There are a large number of reasons this can happen, so it is hard to say what caused your issue.
This is not an uncommon issue, and it can be quite random.
Other than being terribly annoying, I would not be too concerned, unless it regularly happens.
If so, there are help articles for this available in the “Support” tab at the top of every forum page, and a search of these forums will show lots of posts on this, these can contain some good advice on diagnosing/ solving the issue as well.