Completed work months ago, client is stating ownership hasn't paid GC and therefore is withholding payment. Currently are holding contract work and Retainage money.
It is not clear from your question whether you sent a valid and timely preliminary 20-day notice. If you did not, then you do not have mechanic's lien rights. You must also be a licensed contractor. Your question doesn't have much detail, so I can't go beyond that.