I was messing around with the iptables config on my Digital Ocean droplet recently, and got kicked out of my SSH session. It was at that point I knew that I had made some changes that were jacking stuff up.
This Digital Ocean support article gives a nice primer on how to wipe the iptables config bare and start over with a strong base config.
Particularly helpful for me was the command to save this master config to a text file in case it was needed later. To do that:
iptables-save > name-of-file.txt