Someone posts as you: the fix happens in Instagram
For a Threads profile that posts on its own, Instagram's help prescribes the Instagram treatment: a new password, unknown third-party apps removed, two-factor switched on, contact details and Accounts Center checked. Those moves, in order, are on the Instagram recovery page.
Secure the accountA Threads login that fails is an Instagram login that fails
Reset it where it lives — on the Instagram login screen, with a login link sent to the email or phone of the account. Then sign in to Threads with the new password.
The limit If the email on the account was swapped as well, the old inbox comes first: reversing an email change.
Reset the Instagram passwordInstagram's recovery is Threads' recovery
The help draws the line for Threads like this: no access to the email or phone, and the profile not in the same Accounts Center as your other profiles — then access cannot be given. Whatever is still possible runs through instagram.com/hacked, as described on the Instagram recovery page.
The limit What changes when the profile does share an Accounts Center with your other profiles is not described in the help, so we give no steps for it.
The Instagram recovery routeNo route into another person's Threads exists here
And none on the pages that promise one. What this page has is the owner's side: how a Threads profile really changes hands — always through the Instagram login — and where that chain breaks.
How it really happens