I ordered from them a couple of years ago because they had great prices. I was just looking for them now to make another order. When I tried their website I got the message "Bad Request (Invalid Hostname)". I can ping their site, so they are not completely dead.
Ping works on the IP address (hobbycircle.com gets forward-translated to the x.x.x.x format). This only means that the IP address (18.104.22.168) is still alive. If they're hosted by a service, that may mean the hosting service is still up and running, but the site itself may not be.
I did a traceroute (tracert from the command prompt window) and it took
22 hops (internet server connections) to get to 22.214.171.124.
I then tried to bring it up in my browser and got "page unavailable". This tells me the IP address is still set up on internet DNS servers, but the server connected at the other end is not responding to http queries. Conclusion: they're either out of business or they've taken the web site offline for some reason.
I would venture a guess that HobbyCircle has tanked. Like you I had good results with them initially, but about a year ago I placed an order and paid with PayPal and never heard from them again. Well, they did respond to my first follow up with the claim the items were on back order. Then nothing. And nothing, and so on. It's now a year. My guess would be serious financial difficulties lead to closing. Pity, too, they /had/ good prices, but perhaps I now know why.
I checked in with a guy in my class who has a history of hacking and knows a lot more about networks than I do. He said it's likely the IP address is still registered and the name is still stuck in the web DNS servers, but that they probably are out of business, their web site was hosted, and the host simply took their pages offline. That would explain why you can ping the address but not get a web page.