Thanks for the replies, pqglen and sparky. Sorry for the delayed response. I've been offline for a couple of days because my wireless router or modem or laptop, something electronic, is giving me the middle finger!
Anyway, we haven't discussed the situation with our neighbors because they move in and out too fast, but it's a good idea.
Honestly, since I only speak English, I can only communicate with the neighbors across the hall, and they JUST moved in. They're renting. I will ask when I see them again. And as far as the previous owners of our condo, I wouldn't know how to reach them. These used to be rental apartments. They were converted last year. I'm afraid we don't have Applecare, so we can't use Apple's customer support. My husband's Applecare expired a year ago, and I have my extended warranty through CompUSA. Without Applecare, they won't help us. Fortuately, my husband was able to recover the data on the hard drive by putting it in the freezer. It kept it going long enough for him to transfer the data to another drive. What I'm reading on other websites is that most people replace their hard drives without batting an eye because they're inexpensive. We'll replace ours too. Of course, we still won't know what caused it to go bad. Sparky, my husband measured the voltage on one circuit, in the office, where we have all our computer equipment and the light fixture I was telling you about. It read between 119 and 120. Although the reading was normal, he was considering replacing the circuit breaker because he couldn't think of anything else. I forgot to mention that all our equipment is connected to high-quality surge protectors. So you think it could be the transformer? Wow! I guess I'll get in touch with FPL. Thanks for the suggestion. I'll keep you posted on new developments.