In case you renamed a custom web part, make sure you have renamed all the references to the web part: the .webpart file, the .cs and the all the relative methods, and if all is well, navigate to the site collection web part gallery and delete the web parts that do not work and re-deploy your solution.
it is more possible than not that the existing web part (the old one) conflicts with the one you are trying to deploy over it (the renamed one).
How to: troubleshoot an empty SPListItemCollection (CAML Query) How to: add properties to custom web parts