Thanks for replying -- I actually have an update to this: I believe this original issue might have been occurring because I ran out of storage on my EC2 instance (I was at ~99% usage). This would probably explain why it worked before but suddenly stopped working. So I went to EBS & increased the storage (and did the necessary steps to reflect those changes on the EC2 instance as well).
Then, I rebooted my EC2 instance, and this is where things went wrong: now I couldn't even access the Neo4j browser at all; and I'm getting the ERR_CONNECTION_REFUSED issue.
So I ssh-ed into my instance, and found that apparently neo4j was not running.
I have fixed this. I just had to create the neo4j.log file, and I was able to start neo4j successfully. Sorry for being a noob on this; but hopefully anyone else w/ the same issue can look @this thread later.