There may be a variety of causes to this problem. Please carefully go through the following checklist.
When you generate the menu code, a set of supplementary files is generated as well. You must ensure that all of these files are uploaded to your web server and check that they are available on your server after the upload.
If the problem persists and you believe that all of the menu files have been uploaded correctly, you can also check your web page HTML code for errors. You can use the free W3C HTML Validator to do this. The validator will also give you advice on how to fix the errors.
If you have completed the above steps and the problem persists, you can try the following conflict elimination method as a last resort. This step requires some HTML programming knowledge.