Is this supposed to be a shared bike? You co-signed the loan, but did you also put both your names on the title for ownership?
You can enforce a breach of your written contract as a civil, not criminal, matter. The language of that contract governs your respective rights and remedies.
If you're on the title of the bike, she can't sell it without you signing, too. If you're not a co-owner, then she can, but that would apparently breach your contract.
Your loan agreement with the lender is separate - the lender doesn't care what your contract with your co-signer is.
If you're not sure what your contract with your co-signer means and what your loan documents require, see a business litigator for help.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.