Perhaps your transaction hasn't been confirmed? This would mean you see the pending balance in your account, but can't yet spend it as it hasn't been fully verified.
Could you share the public address of your wallet, so that we can check the status?
Also, what wallet software are you using?
Could you please let us know, what site you withdrew from?
As @CryptoBlackHat posted, we will also need to know what wallet you are using and the receiving address you used.
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