My domain has been transferred to some other registrar without my knowledge
A registrar may only initiate a transfer if it has obtained a completed Form of Authorization (FOA) from either 1) the registrant or 2) the administrative contact for the domain. The registrar must be able to produce a copy of this documentation when it is requested. Failure to do so is grounds for reversal of a transfer in the event that a complaint is filed under the Transfer Dispute Resolution Policy.
Thus, the first step should be for your previous registrar to request from the new registrar a copy of the form used for authorizing the transfer. The registrar can then proceed with initiating a dispute as appropriate. Contact your previous registrar for assistance.
If you believe that your domain name was transferred to a new registrar without your authorization or consent, please contact the original registrar. Disputes between registrars over alleged violations of the Inter-Registrar Transfer Policy may be initiated by any ICANN-accredited registrar.