I was doing mining with Bitknock but they closed the website without informing us.
I can't access my wallet. Here is my wallet address
What must I do now? Should I acknowledge that I lost my Bitcoins?
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Unfortunately, most of the "cloud mining" websites are ponzi schemes trying to disguise themselves as a cloud mining contract provider.
This site catagorizes Bitknock as a ponzi/scam dated back to November 2015. A little bit research could have made you aware of the dangers.
I personally lost dozens of bitcoin in such projects, even if I joined only the most secure looking. General rule of thumb: If it looks too good to be true, don't buy it.