As speculated, Prince died from an opiate overdose. He was rumored to be addicted to pain killers prior to his death. He’d even reached out to Dr. Howard Kornfeld, a national authority on opiod addiction treatment and had planned to enter a rehab program, but died before that could happen. Source and Image
Prince was in pain for years but didn’t want to stop performing so he treated it with opiates. It’s sad that his pain won out.