You've probably only got one circuit in the loft, and the window air conditioner is sucking too much power so the circuit is starting to overload. First, plug in a light to every socked you've got, and have someone else up there while you start flipping circuit breakers on and off. If they see some but not all of the lights go out, you know you've got two circuits and you just need the AC on one and everything else on another.
If you only have one circuit, if you want to fix it the right way call an electrician and have another circuit run up there just for the AC unit. With a heavy enough extension cord you could do a temporary fix by plugging into another circuit downstairs, but since an AC unit generally has high amp draw you'd need something heavy like a 0 gage or 0/0, and that's a lot of extension cord. The only other thing you could do is change out the circuit breaker from the 20 amp (which it probably is now) for a 30 amp, but then you risk overloading the wiring and causing a fire, so while this would proabably work it's not an option I recommend.
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