We has a form that mysteriously was closed. This was unnoticed for a number of days before the author reopened it. This form was not closed by the author.
What could have caused this? Any events that may have closed this form? Are there any locks or safeguards to prevent this in the future?
The only reason a form would be automatically closed would be when the response limit has been reached, you should check the View Responses table and if you are at the limit you can export responses to Excel or CSV and then delete existing responses to free up space in your form, then you can open it again to collect more responses.
Another possibility, are you sharing the form with anyone? Are there any Co-authors who could have closed the form accidentally and not re-opened it?
Note that most changes that you make on the Design tab, and some on the Options tab will alert you that the form will be closed to make that change - it is very easy to make an edit and then forget to re-open the form. The most common cause of a form being closed unexpectedly is that someone made a change and did not re-open the form.