There's no way to reconnect. I've not yet seen an definitive cause, without which there can be no definitive solution. It just seems to happens sometimes, no one knows why.
Having said that, you often don't actually need preview files and are usually encouraged to export without using them.
This is a pretty intricate project with red footage with effects so I need to preview it with the effects. We've got a down-resed proxy file for preview purposes but it's frustrating to lose those renders files after an all night render.
This is an issue that Adobe is acutely aware of and wants to help us solve. Please fill out a bug report that is as detailed as possible.
I spoke to an Adobe staff member about my experience with this recently so they are aware and motivated. Its just in my expereience its a very random occurence.