Desktop Prezi not letting me edit existing Prezis

HELP!
I have downloaded Prezi, referred three friends who all signed up. I then downloaded Prezi desktop and even bought a prezi online for me to edit. Unfortunately, I have just saved and closed my Prezi and now when i reopen it, I am unable to edit it...
Why am I not able to use Prezi Desktop?
If I cannot use the desktop version, am I able to upload it into the web version?