This urgent project might have done better without you. The company might royally fuck it up. You shouldn't feel guilty for leaving a company, because a company certainly won't feel guilty if they had to get rid of you.
Like others have said, these are the sort of conversations that are best had face-to-face. There are countless reasons as to why they've not emailed back, but personally I wouldn't dwell on it. I'm sure they'll be fine, and I'm sure you will be too.
We all grossly over estimate how important a single individual is.
Anyway, what's done is done. Don't worry. If they really cared, then they would have responded to your email and explained the upcoming urgent project. It's even possible they were wanting you to leave. You spared them the pain of firing you to make room for a new person.
Good luck with finding a job where you are better appreciated and given the extra responsibilities that will let you showcase your skills and dedication.
I left my last company after a year, right after I managed to push one of my brain child through. It was a bit awkward because of that. But they recovered and so did I.