In order to avoid loss of data in case something goes wrong - like a power-loss or application crash - BJSurvey saves the data you are entering to its database every time you complete a round. Therefore, the maximum you could lose is the round you are currently working on. BJSurvey has always done that, but in v11 I removed the "Save Evaluation" function from the "File" menu to simplify the options.
When something does happen, just restart BJSurvey and load the evaluation you where working on. If you entered the "Play Date", the evaluation will be listed close to the top of the list of evaluations. Alternatively, you can search for the player's name - if you entered it. You can always sort descending by "FileId" to bring the evaluation you entered last to the top.