I'm so sorry to hear that. As a part of the Accessibility features on mobile devices.
There is a thing where if you hit the Enter or Return button while entering data in a data entry field.
The enter will actually be applied to the Done button on the checklist, the result is that it will submit your checklist.
This doesn't happen in text fields, like comment boxes but it does happen on other data entry fields.
The Bad news:
We can't do anything about this because it is built into the Operating Systems to help people with vision problems.
Your checklist cannot be reopened, it has been submitted.
The Good news:
If you just click out of the box and don't hit the Return or Enter buttons, this won't happen.
You can add a required True False question to the bottom of your checklist, ex: "I'm ready to submit" and because you would answer it last, it would stop you from submitting accidentally.