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If you are a paying customer try zendesk support as you'll probably get better service.
But in the meantime, I can help you out. Are you able to view these prezi's? http://prezi.com/explore/
If not, its probably a problem with flash or your firewall. Download the newest flash here http://www.adobe.com/support/flashpla...
If that doesnt help, you'll need to find out what security system you have and whitelist prezi. Often times if you are at work or a school, you'll have to request to have prezi.com whitelisted by the IT guys. If its your own network, try googling "(your security/browser system) how to whitelist"
Hope that helps!