If you lose your BRP, you must apply for a replacement BRP within the regulated time. You may be fined up to £1000 and have your permission to stay could be removed if you do not apply within time.
If you lost your BRP in the UK , you must apply within 3 months from the date you lost your BRP. A replacement BRP costs £45. You must report to the Home Office immediately by emailing BRPLost@homeoffice.gsi.gov.uk, calling +44 (0) 208 527 3263, or via online report (https://www.gov.uk/biometric-residence-permits/los...）. You can choose either postal or premium service (only if you have leave to remain or indefinite leave to remain). The application process and details could be found here:https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/419177/BRP_RC__guidance_notes_04-15.pdf
If you lost your BRP outside the UK, you must apply within 1 month from the date you lost your BRP. Again, you must report to the Home Office immediately by email, phone, or online report. As BRP cannot be applied from the outside the UK, you must apply for a single entry visa to re-enter the UK. Once you entered the UK you will need to apply for a replacement BRP as explained above.
BPA assists your re-application to enter the UK from outside and replacement of BRP. Please do not hesitate to contact us.
(Ref: https://www.gov.uk/biometric-residence-permits/replace )