I have a Gun Smoke board that at first gave a white screen with a few odd squares in the corners. I reseated all the chips and cables, and then got a plain white screen.
Check the roms were reseated correctly, sometimes legs bend under the chip. Also you should put the roms through a rom reader to verify the game data is correct before attempting anything else - just the code roms should do. If that fails make sure all the tracks for the code roms are not broken. When boards fail to boot completely it can often be that the game code isnt reaching the CPU intact - you'll need to verify all the address and IO lines are ok. Of course you could be lucky, might just be a dead crystal (if you dont have on oscilloscope use a logic probe on pin 6 of the main CPU - a Z80 on the top board that isn't near the sound section, on Gunsmoke) - if it isn't pulsing the clock line for the CPU is dead, trace back and find out where this dies.
Welcome to your baptism by fire. Same thing happened to me with an Up n Down board that caused me to learn basic logic electronics...