Youtube – Combine/merge long YouTube videos

You can't without becoming a partner.

The upload limit on YouTube videos has been increased from 10 minutes to 15 minutes. One of the reasons we can increase the video time limit is that we've made incremental improvements to our Content ID technology that helps content owners manage their videos on YouTube. We look forward to increasing the upload limit as Content ID continues to evolve and become even more sophisticated.

If you want to upload videos longer than 15 minutes, try another site such as Vimeo, Blip.tv or Viddler. You could also host it yourself using JWPlayer and a CDN such as CloudFront. As Even Mien said in the comments, you could also see if you can become a partner.

YouTube now allows user to upload videos greater than 15 minutes; but it was not so a long time ago (when you tried uploading those videos).

For reasons known only to Google and YouTube developers, the info (Title of the Video, Description, Date and Time of Upload, and may be even a Screenshot of the Video) of the videos that failed to get uploaded to YouTube (due to longer duration or unproper file format)(or were uploaded but later removed due to Copyright Violation)(or were later deleted by the up-loader itself); still remains in the User's accounts and only the particular User and the site's moderators/administrators can see them. If a link for the video was generated and anyone happens to visit the link, there would be a message saying that the Link is invalid or Video is deleted or Video removed due to Copyright Violation or something like that.

You cannot Combine, Trim or add soundtrack to those videos as they were never uploaded at all, or deleted or were uploaded and still are available with Youtube but no one has access to them as someone filled a Copyright Violation Complain and Claimed it to be their property.