Why is my withdrawal from the 50BTC pool not visible in the blockchain yet? I did a bitcoin transaction of 216 BTC from the 50BTC mining-pool to my wallet (Bitcoin-Qt).

It has been 3 days now and I have not received anything yet.

I've tried to search my address on blockchain.info and it doesn't come up.
50btc was hacked some time ago and the hacker completely messed up all account balances. If you actually have mined over 200 BTC there, I suggest you contact the administrators. But I assume you're just one of the people who woke up one day and saw that your 0.001 mining profit had been replaced with a completely arbitrary number of coins, in which case no, "your" Bitcoins were not lost since they never existed in the first place.