WHAT TO DO WHEN AN OUTPUT SIGNAL IS NOT COMING FROM THE PLC?

f the 24v is connected to a external relay could could test by disconnecting from the PLC and applying the coil voltage. Take care other safety features would not be over-ridden. If you can obtain a copy of the software you would at least have a listing, even without any other documentation it would be helpful

in troubleshooting the plc one need to confirm if the problem is internal  to the processor or in the I/O system

MAY BE A24V IS EXPECTED TO SWITCH A DEVICE AND IS NOT COMING FROM THE PLC

u better check physical connections of ur plc, check your out put & input with the DMM, its very rare to loss the output when input is available.