To retrieve lost Aadhaar online, visit the UIDAI website and retrieve your Aadhaar number or Enrollment ID by choosing 'Retrieve Aadhaar number/EID/SID'. Enter your name, registered mobile number or email, and captcha, then verify with the OTP. Next, go to 'Order Aadhaar PVC Card' and order a reprint for ₹50. Finally, track your application using the Service Request Number (SRN).