I see this is an old question, but decided to post an answer anyways.... Likely, if a Rena/Filstar Filter doesn't start back up after a power outage, it just needs to be primed. We have had this problem many times (but fortunately, we have a generator so the fish are not without their filter for long). To prime, unplug the filter, release the disconnect, take the little cap off the top of the intake tube and pour a little cup of water into the tube using the small funnel that came with the filter. I have a little plant watering can that works perfectly (with a long pour spout). Put the disconnect flap back down and plug in the filter. If it doesn't start back up, repeat the process (be sure to raise the disconnect flap again).
I have have ten year old Rena xP3....Love the filter....When I needed a filter for my 20 gallon, I bought an xP (smallest).