Recently when my PCR reaction was running there was power fluctuation and the entire lab was blacked out for a few minutes and unfortunately PCR that I was running got switched off. So, would it be okay to restart the PCR? What would happen if the same PCR program was run again?
The polymerase used in PCR is extremely stable at room temperature. I would expect you to be able to continue your PCR reaction from the cycle it was stopped at without too much trouble. However, it is important to not over cycle your product. This will produce non-specific products. If a clean result is imperative, take a 1-5ul sample from your original reaction to use as a template in a new reaction.
In this case you can run your PCR for another 18 cycles (if your total number of cycles was 30). However, do the initial denaturation for 1-2min. You may run the same program again (full 30 cycles) but you'll just end up wasting time because after a point all the dNTPs and primers would be exhausted and further cycles will have no effect on the concentration of the amplicon.