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Are any of the devices plugged into a power strip at all? For example a 4 way extension that plugs into one socket on the wall?
Does this happen often or is this the only time? It could be a power fluctuation on your mains because the electronics inside the computer, modem and tv are sensitive enough to register even a split second drop in power at which point they reset themselves.
It's possible that changing a setting in the BIOS of the PC will let it boot up after a power outage without you having to switch it back on manually.