The procedure to recover an openwrt you have lost control is to try the recover mode, as it will boot will a minimal configuration, a known IP, and depending on the version of open-wrt can be accessed via telnet or ssh.
You have here the links to the considerations of the procedure https://wiki.openwrt.org/doc/howto/generic.failsafe and a youtube video talking about it at https://www.youtube.com/watch?v=axQvgqvlhcU
Basically you reboot it, and when one of the leds is blinking in a particular pattern you press the reset or the power off button. The appliance will then boot in safe mode with the default IP address 192.168.1.1.
As we have seen in your case, it will then be enough to ssh to it as root with no password.
As for people thinking what is openWrt, it is a Linux-based open source substitute firmware for the standard image that comes with some models of routers that happen to have hardware compatible with Linux and that in some cases have placed their firmware or parts of their firmware in the domain of open source.
The firmware gives you more control, and often will also provide extra functionality. In my case, the default firmware does not allow the use of the VLAN chipset for more than the standard Internal/external network at fixed ports, whilst with openWRT the VLANs can be programmed at will, and extra functionality can be added with standard Linux software.
I will leave here a link about the differences between openWRT, dd-wrt and Tomato.