A spokeswoman for Michaels confirmed reports that the 47-year-old musician was rushed to hospital overnight with an excruciating headache and was in critical condition.
Doctors diagnosed a massive subarachnoid hemorrhage, or bleeding at the base of his brain stem. The rock singer's spokeswoman said there would be no more information given until he has additional tests.
Celebrity website TMZ.com reported later on Friday that Michaels was awake and talking. TMZ said it spoke to the woman who won a dating reality show with Michaels in 2008 and who was in contact with those close to the singer.
Michaels was in intensive care and will be closely monitored for the next 48 hours.
Michaels rose to fame in the 1980s at the helm of Poison, a glam-rock band whose hits included the ballad "Every Rose Has Its Thorn."
He was diagnosed with diabetes as a child and just last week disclosed he had an emergency appendectomy earlier this month. In a blog entry, he called it a "wake-up call to be a little bit more diligent on keeping control of my health."
In recent years, Michaels found fame as a reality TV star in such shows as VH1's dating contest "Rock of Love" and on the current season of "The Celebrity Apprentice."
His latest album "Custom Built" is due to be released later this year and he recently recorded a song with Disney starlet Miley Cyrus.
On "The Celebrity Apprentice," which was filmed several months ago but is about halfway through its broadcast run on NBC, Michaels has raised $100,000 for his charity, the American Diabetes Association, and is a strong contender to win.
"Apprentice" host Donald Trump said his thoughts and prayers were with Michaels.
"He's a great competitor and champion and I hope he will be fine," Trump told celebrity TV news show Entertainment Tonight.