If your Sub/Subsequent 37 is stuck and the screen is blank and/or all LEDs are illuminated try the following:
First, try forcing firmware onto the synth via the manual install process detailed here for Sub 37 and here for Subsequent 37. Within your sysex program, if available, click Preferences-Transmit Speed and lower the speed to 30%. You will not be able to see the prompts on the screen with the synth in this state. Simply proceed to send the next file after you see that your sysex handler has finished sending the current file.
If the issue persists, try reseating some of the internal connections. To do this, remove the two screws on each side piece that hold the front panel to the chassis. Lift the panel up and away from the chassis and look for the ribbon connector that travels from the left panel board to the right panel board. Disconnect and reconnect this cable. Similarly disconnect/reconnect the connector that travels from the left panel board to the bottom board. It's ok to remove any silicone that's present around connector heads. Once complete, power on, and if the issue still persists, try another firmware reload