Normally they have to be sent back to a home or safe position in manual from the flex pedant/teach pendant before they will boot up, Its worth running a home routine if you can get the errors to clear. You'll need to see what the error code is before you can do anything else really.

What was the arm from previously? Factory/production use? If so the controller might be configured to wait for an external input (safety gate/run signal from a PLC etc)... even if its waiting for an external influence you should still be able to step through the previous code or jog the robot in manual from the pendant... you might have a keyswitch input on the conroller that over-rides the inputs allowing it to run in manual, then you'll probably have a deadmans handle on the pendant? without this held in, it wont move.

Connect it all up and see what happens/see what code you get, Just beware that it could start up at any moment and make sure its bolted down to something solid before you turn it on.