No reply after declining a PhD offer Couple of weeks ago, several professors from one of the universities I received an offer from wrote me asking if they could help me with anything (to decide on a place I choose). As pleasant as it was, I wrote (what I think was) a very nice email declining that PhD offer to each one of them. It's been quite a while now, but none of them have replied, while from other places I've received wishes of good luck. Have I burned some bridges there?
Probably no burned bridges. Like everyone else (especially now) they are busy and just focused on the essential things they need to do to keep everything else together. In normal times you might have gotten a few more replies, but not from everyone. They just say to themselves "oh well" and move on to the next crisis. 

I suspect that even those who haven't replied hope you succeed. When one does, we all do.