I would first suspect the power supply. Series 3's are well known for getting hit with capacitor plague
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Your power supply could already have been suffering, and the power outage was the straw that broke the camel's back.
The Tivo operates with three different voltages, so I would think it is possible for some things to work while others don't in the event of a power supply failure.
In your case, maybe the tuners aren't getting the voltage they need to work.
Open the Tivo and visually inspect the capacitors. If they all look completely flat on top, then move on to measuring the voltages of the power supply's outputs.
Use the search function of this forum to find other threads about this. There are a few that go into much more detail.