how do I put on screens if I no longer have access to the platform?
I know, it puts you in a pickle. Most cryptocurrencies use public blockchains where transaction data can be verified by anyone. It's an invasion of your privacy, but you could show records proving you deposited those coins to an address/addresses connected with that Decoin exchange. Your receiving address on Decoin would, of course, have to be connected to a known Decoin address by some service like WalletExplorer, Breadcrumbs, or similar.
I'll look at my transaction history and find it for you! it's not the problem, just a bit of research to do and I'll send it to you in a few days!