Anushka23 - I was in a very similar situation with my first guest! My power outage, however, was due to a tripped circuit breaker, and only affected 2 of 3 bedrooms. My heart literally dropped!!
I was able to report the outage to my guests on the morning prior to their arrival, and they Graciously said that they still wanted to come. We were able to provide a work-around with Heavy Duty Extension cords to provide power in those areas. Otherwise, they loved their stay.
My takeaway is to Communicate Early, and Often; Be Honest, Empathetic, and Transparent; and Follow up. People, as we all, just want to feel appreciated.