What the others said above, and also:
Try to reproduce the various states which were described.
-If you connect it to a TV via an RF lead out of the modulator, and tune it in on the TV, does it boot up OK and work?
-Leave the RF connection in place and the TV on, temporarily power down the STe, plug the monitor lead plug into the STe's monitor socket but do not plug the other end of the monitor lead into the monitor / TV. Turn on the STe, what happens, does it still boot normally, with the picture on the TV (via RF) looking OK?
If the answer to that was yes, now, power down, plug the other end of the monitor lead into a video monitor or TV - it can be the same TV you're using for monitoring the RF output if you only have the one TV, power back up - still watching the image being output via RF, does it still boot OK?
If the answer to that is also yes, now switch to the AV / SCART input on the TV/Monitor. If there's a picture, try rebooting the machine again. Does it boot back up OK?