If you have been refused a visa in the past then it's you who has been refused, not your passport. It's irrelevant whether you have subsequently changed passport, whatever the reason.
If you have failed to disclose that refusal in a subsequent application you now have a problem. If the country to which you are now applying knows about that refusal but you fail to disclose it they will deny you entry and possibly impose a long ban for deception.
If you do now disclose the previous refusal, the country to which you're applying will want to know why you didn't disclose it before. If you don't have an excellent explanation they will deny you a visa and possibly apply a long ban for deception.
From the tags and your comments it appears that you are applying to the UK, having been previously refused by the US. Those countries share a lot of immigration information. You should assume both countries know everything about your visa history.
Your history of visa refusals coupled with a failure to disclose previous refusals makes your situation complex. You would do well to consider employing a UK lawyer with immigration experience if you seriously want to visit the UK.