You need to provide a little more information. What exact year and model of Twin do you have? Can you post the schematic?
There are a few very basic basic steps you can take that can be very telling.
Turn the amp on and let it warm up for a bit. Observe the tubes when powered up. Make sure they are glowing (but not cherry red). Record DC voltage on all pins on each tube and list them here. Start with V1 and make your way over to the power tubes.
You mentioned that you have no voltage present when you measure at the bias test point. This indicates that either your bias test point is bad OR that your tubes are not drawing any bias current.
I am just thinking of how I would start to break it down if it were mine.