504 Gateway Time-out when Activation

As instructed I installed the activated theme on my wordpress and after that I used the key on my dashboard in 'Theme activation'. After using the key, the page will load for 1 minute and then it says "Gateway Timeout 504". I've tried entering the key for 5 times with no succes.
And now it gets worse: Now I am not even able to connect to my website anymore. It almost looks like it's gone or something. Even the FTP folder of my website is not accessible anymore. This is a really bad problem because I might lose customers on this problem.
My friend however can see my website without any problems. So my conclusion is that I am banned from my own website. Is this correct?
Please help me to activate my website (possv.com) and make it go online again. I'm taking financial damage here.
Kind regards,

Hi!
Please contact us via support@smthemes.com to solve it.