If the Deal Loss Reason screen no longer appears when moving a Deal to Lost, no worries! You may have selected 'Don't ask me to specify deal loss reasons here again' when moving a Deal to the 'Lost' stage in the past - restoring this to view is as easy as following these three steps:
1) Go to Settings
2) Select Deals
3) Click Loss Reasons and check the 'Ask For Deal Loss Reason'
This will resume prompting you to select a Loss reason when moving a Deal to your Lost stage.