If there was no contract, then there would realistically be no consequences. However, if you received a sign-on bonus it is likely that there was a written agreement of some kind before you received those funds.
If there was an agreement we are unable to answer your question without first reviewing that agreement.
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You will almost certainly have to return the sign-on bonus. As for whether the employer has any other options, it's impossible to tell without having a local employment attorney review the actual documents.