While there was plenty of summer fun to be had at country-music concerts coast to coast this weekend, fans were no doubt disappointed at a couple of medically-related cancellations.

At Kenny Chesney's Sun City Carnival stop in San Francisco on Saturday (July 18), Sugarland was a no-show due to lead singer Jennifer Nettles' lingering throat problems. The San Jose Mercury News reports that signs posted outside the venue informed concertgoers that Jennifer had lost her voice in the recording studio. The upside: longer sets from opening acts Miranda Lambert and Lady Antebellum.

Jennifer and singing partner Kristian Bush posted a YouTube video apologizing to fans, explaining that Jennifer has been dealing with vocal issues throughout the year. "I sang too much, I blew my voice," she says in the clip, adding, "It's like a muscle. It's like another part of your body. If you sprain it or strain it, then you have to heal it or else you will break it. And I don't want to break it because I want us to come back out there and see you."

In spite of the setback, Kristian notes that Jennifer has maintained her sense of humor. She assures fans she's not sick or dying, and jokes that instead of telling the truth, she thought about revealing that she'd been binging on opium and absinthe in Paris, or that she was attempting to develop a "robot voice" to score a role in the next 'Transformers' movie."

Another artist who's had his share of ongoing health issues, Merle Haggard, has postponed or canceled a handful of shows, including dates this weekend at the Smoky Mountain Fan Fest in Sevierville, Tenn., and Jamboree in the Hills near Wheeling, W.V.

CMT.com reports that Merle is suffering from severe head and neck pain. The 72-year-old was diagnosed with lung cancer last year and had surgery to remove a malignant tumor.