In hardware configuration double click the CPU module. Under the "Settings" tab look at the "Power-up Mode". Your choices are Run, Stop, or Last. Default is "Last" and most people see no reason to change it. That means that if it was running when power went off, it will come back up running.
If yours is set to "Stop", you can change it back to "Last" or "Run", but before you do, make an attempt to find out why somebody decided to change it from the default setting.
If yours is set to "Last" and you are typically powering up in the stopped state, then check the fault tables for both the I/O and the CPU. There could be a fault preventing it from going into run mode.