Omg I'm going through the exact same thing now. Been talking everyday online for 3 months, things were going great, we had plans to meet and suddenly out of the blue an odd facetime. The next day he drops a bomb about spiraling into a dark hole (manic depressive) and says he needs to dissappear for a few days.
There were no signs at all. He did mentioned that he was recovering from depression but seemed fine as we spoke. Reading from all the comments and past experiences here tells me that I have to start moving on myself to protect my own heart. It's sad because I finally let my guard down unknowingly after 7 years of an eat pray love journey just to be hit by a truck out of the blue, without knowing what I can do or did.