if you have checked all the leads, try checking another power supply on the board, sometimes the PSU on Promethean boards drop voltage and the electronic control pack won't fire up.
Also if your board, is a far distance from the controlling PC/laptop and it is serial connected, then check to see if the installed used a Serial Lead extension, if they did check the connection.
If it is USB connected check the power connection lead (usually behind the board) hasn't come adrift.
If all else fails try removing the connector from the back of the board, turn off the board power supply, wait 2 Minutes, then reconnect the board and turn on the power.
The Electronics packs on the boards are very senistive and usually blow if subjected to too many power spikes of outages
