You can't link/merge accounts, all content will remain tied to the account that downloaded it.
Why can't you access the account ? If you've forgotten its password then you can reset it via http://iforgot.apple.com e.g. by answering the account's security questions.
If you can't get it reset via that page, and you don't have a rescue email address on the account for the reset email to go to, then try contacting Support in your country, they should be able to reset it for you.
Contacting Apple about account security : Contact Apple for help with Apple ID account security
If your country isn't on that page then try this form to contact Support : https://www.apple.com/emea/support/itunes/contact.html
When you've got access to the account update the primary email address to it to an email address that you have access to : Change your Apple ID - Apple Support
After changing it : What to do after you change your Apple ID email address or password - Apple Support