If you don't have the details because they've moved, ceased trading, you've lost them or whatever then that doesn't affect that you've worked there so I'd still put down the last information I had for them - it does however make it a bit awkward if your potential employer wants to contact them for some reason. Provided they aren't within your past two or three employers (or directly relevant to the current job you are seeking perhaps) I doubt it'll matter too much as many places will only ask for two referees.