There are a few common reasons why this can happen:
If you stay on one page for an extended period of time without saving, your account may “time out” without warning.
A weak internet connection may momentarily disconnect your computer while you are working on the application.
As a safeguard, we recommend that you:
Save your application often
Cut and paste your application answers after each question into a Word document to save as backup.
To restore your edits, try:
Re-loading your internet page, as sometimes the browser will cache an older version of your page.
Logging out, wait a few minutes, and then log back in and re-open your application.