Normally they won't loose any data, because in their browser they still have all the data in the form

What you can do on the server side is ensure that when they try to submit that data after their session has run out, that that data gets accepted after login. So they get asked for username and password again and if that is correct either the server has remembered the form data and submits it automatically or they get forwarded to the page they had been with the form data they had at that time filled in. Then they just have to klick the submit button again