Hi I have had this machine for 9 years, The first time this happened to me I put it in for repair only to be told nothing is wrong with it. It happened again two years later. Like you I tried everything, left it alone for a day then I remembered what the repair man said about it had nothing wrong with it. This made me look at the machine, things like if you lift up the lever it takes the tension off so you can pull out the thread. I turned the handwheel and this released the FEED DOGS works fine again now. This is what happened to you I think, you did not realise you turned the wheel thats why it started to work again with out you realising what you did. It will happen again at some point so now you know you just have to turn the wheel. Gwen uk

Have had my 6600 for 9 years and this has happened to me only a couple of times. Mainly when I drop the feed dogs to free motion quilt. When they don't come back up, I just turn the machine off, wait a few minutes, turn back on and it is fine. I agree with Gwen turning the handwheel will release them too.
Hope it behaves itself while you are at retreat. Most perfect machine I have ever owned and if it gets a little temperamental now and then, it is OK with me!
Have a fun retreat.

great tip, thanks

I read this with great interest, as I have the same machine. I hope you will post the solution when you find it. I haven't had the problem occur...yet. Hope it doesn't. I can only say, "Don'tcha just LOVE this worker bee machine???"