Although the band did not disclose details about Armstrong's illness, reports claim it had something to do with Armstrong's throat.
Armstrong's bandmates, Mike Dirnt and Tré Cool, released a video statement to offer their apologies to fans for canceling their show.
"We apologize profusely," Dirnt said. "Billy got rushed to the hospital last night due to illness. The doctors don't think it's a good idea for him to play today."
"We're beyond devastated," Tré Cool added. "We're as sad as you guys are. Billie's extremely upset about it, and we'll see you very soon."