I'm sorry this is happening to you, but I'm going to give advice, as a former contractor (landscaping, not roofing), to everyone reading this:
NEVER tell a contractor "no hurry" or "it doesn't have to be done right now". Nearly every contractor bites off more than they can chew, because it's hard to say no to money. They will almost certainly put you at the bottom of their list if you say that.
Personally, I never took money up front, so at worst, when I took on more than I could handle, I'd just end up saying "Sorry, I don't have time to do it." after a few weeks. It didn't happen often, but I plead guilty.
Good luck, and keep us updated.