When the reduced engine power light comes on a fault code will be set in the powertrain control module. Reading the code and following the releated diagnostic information should help lead to the cause of your reduced engine power light coming on.
I just spent $200.00 for a new alternator...after no relief I remembered My ski doo..when the rider plug was in the half connected position it sputtered and would not rev up....Look on floor..go to the gas pedal...wiggle the wire bundle at the plug and try the engine. I did this and it cleared it up instantly.....expensive lesson to learn.....
Poor connections at the gas pedal or the throttle body are always a possibility when diagnosing "reduced engine power faults". If the connectors are loose or damaged and cannot be properly secured or the wiring is damaged then appropriate repairs should be completed to avoid reoccurring problems.
I have had this happen twice with my 04 Trailblazer, once last winter and again today. Both times it was at least 20 below zero so it definitely has something to do with the temperature. Both times I let my vehicle run for a bit and then I shut it off and started it again. It works fine after that. After the first time I took it to a dealer and they couldn't find anything wrong with it.