O'Connor was reportedly found at 1.45pm local time on Monday.
The news that O'Connor was missing was announced in a press release from the Wilmette Police.
The release said: "The Wilmette Police is seeking to check the well-being of Sinead O'Connor. O'Connor reportedly left the Wilmette area for a bicycle ride yesterday at 6:00AM and has not returned. A caller has expressed concern for her well-being and no other information is available at this time."
Wilmette Police Chief Brian King said O'Connor was riding a Raleigh motorised bicycle with a pink basket, and was last seen wearing a black parka, black leather pants and a sweatshirt with 'Ireland' written on it.
Before she disappeared on Sunday, Sinead posted a message online directed at her oldest son Jake Reynolds about her other son Shane, 12, who has been placed with TULSA child services agency in Ireland.
She wrote: "Jake, kindly go to the court on Tuesday and take custody your brother from Tusla. My lawyer will be making the illegal way yourself and Donal got him into Tusla (lying to the cops etc) known to the judge,' wrote O'Connor.
"Expect to be in trouble. In fact you'd best bring a lawyer of your own. And do not abandon your brother or any other of my babies again. What you have done to your brother and your mother is LITERALLY criminal (sic)."
The Donal she refers to is believed to be Shane's father Donal Lunny.
And just days before that, she posted an emotional message to Shane, which read: "Baby, I've been trying to get you out of care but Tusla are being monsters. I have to back off because they are hurting me so badly I get unwell again if I go near them.'
"The best thing I can do is advise you get a solicitor of your own which you are now entitled to because you are 12 and bring a case of your own against Tusla, under the Children's rights clauses of the constitution (sic)."
Where to get help:
• Lifeline: 0800 543 354 (available 24/7)
• Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
• Youthline: 0800 376 633
• Kidsline: 0800 543 754 (available 24/7)
• Whatsup: 0800 942 8787 (1pm to 11pm)
• Depression helpline: 0800 111 757 (available 24/7)
If it is an emergency and you feel like you or someone else is at risk, call 111.
- BANG! Showbiz