Try to avoid this situation at all cost. After all, you must be able to show a valid ticket to the train attendant at any time. If your battery was empty, it is best to buy another ticket at the ticket machine on the train platform. If you do not have a valid ticket, the inspector will prepare an irregularity statement and you will have to buy a ticket at the on-board rate. This with a supplement of € 7.00 on top of the normal purchase price.
You can then request a refund within 14 calendar days (including the date of the check) at a NMBS counter. If you can prove that you had a valid ticket in the 4411 app, NMBS will arrange a refund. In this case, also take your irregularity statement with you.
More information can be found on the NMBS website (www.nmbs.be).