It is likely that this has something to do with the transaction congestion on the Bitcoin blockchain right now.
Your coins are probably not lost, just the transaction is unconfirmed and you will probably need to wait for this to fix itself eventually.
You can also contact Changelly and tell them your issue, they might give you more information on this.
Topic: Unconfirmed exchange of bitcoins (Read 178 times)
I exchanged a bitcoin amount for bitcoin cash by Changelly yesterday at 12/21/2017 but until now the exchange has not been made. Did I lose my bitcoins? 
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