I was dumb enough and just executed:
iptables -A INPUT -j DROP
This blocks everything of course :)
Is there any hope for me to somehow still connect to the server?
You might have to either get access to the console, have someone else use console access, or reboot the server, assuming nothing saved your new iptables setting.
Use keyboard and screen, serial port, remote KVM, virtual serial port to VM host, or some other such recovery mechanism. Otherwise reboot.