#The spellcasting is disrupted
This is essentially like counterspelling yourself. You have started casting a spell, but then while you were doing that something happened that prevents you from casting the spell.
If you aquire a status that prevents you from taking an action while taking it, you fail to complete it. Lets say you are rudely interrupted by an arrow that was readied to "when the caster starts casting" and fall unconscious. You do not then finish the spell. You have aquired a condition that would prevent you from taking the action, so it is now disrupted: started but not completed.
The answers to this question about the stunned condition are also in line with this interpretation. (Thanks to Medix2 for the link.)