Hi there
I've got an rb800 which I believe has locked up. I can't ping the rb800 or winbox to it.
It's connected to a computer which I have remote access to from another LAN port.

The port one computer which has the rb800 shows as it's still linked but with an invalid ip address. Sadly the serial cable isn't connected.

Is there anyway I can issue a reset by LAN?

MAC telnet or MAC winbox if you have other equipment connected via layer 2 that can run as a client for those protocols, and haven't turned off the service on the hung board.
Since a computer is directly connected MAC winbox sounds feasible. Run winbox on that computer, click the "..." and see if a neighbor populated, click on the MAC address, provide credentials, and connect.

And plan a trip there to connect the serial cable. Whenever you have equipment with console ports and equipment you can connect the console port to, do so as long as you can still reasonably ensure device security measures as required. No one has ever been on their death bed saying, "oh my, I had too many out of band connections to fall back on".

I've tried to find the MAC address of the unit, but sadly winbox isnt pulling any information about the unit.

Normally I would be able to go to the unit and sort it out, but this time round I'm in a different country :/

Then layer 2 access is turned off, or somehow blocked - in which case you need physical access.

Or you can try sniffing traffic on the router facing link to see if you can find the IP address (you simply stated it was "invalid") and re-configure the computer to be on the same network on that link to gain access via layer 3.

Thanks for your suggestions, but given the setup, my only options is a physical connection to the unit