I do not know anything about poker/slot machines, but I do know computers. I am thinking that your poker machine was previously used in a casino where it was hooked into the casino's computers that track and monitor play on the machines. I am guessing that the machine is still set up to play in "connected" mode to a computer. That is why it is telling you to "check signal cable" since it is not connected. The signal cable is what connects the machine to the computer through a local area network or LAN. There is probably a way to set it up for "stand alone" mode where it is not connected in a computer. I would call the manufacturer and ask for their tech support department, or you might even find them on the internet, and ask: 1) What does the error message mean; 2) How to open the machine up without keys; and 3) How to set it up for stand alone mode.