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.
If you look on the Mining page at 50BTC, you will be able to see how many coins you have actually earned in their pool (because, as Yngve mentioned, 50BTC was hacked at the end of October, so the account balances are inaccurate).
By subtracting the number of coins that have been paid to your payout address (use a service like Blockchain.info), you should be able to determine exactly what is owed.
I have reached out to them a number of times because I'm still owed 7.4 BTC and have had no response yet. Best of luck to you; let me know if they reach out to you!
Just to double check, did you double check the address to where you sent the bitcoins. I know, you were suppose to send them to yourself, but could you have made a typo and sent them to someone else. If that was the case, I am afraid you made someone rich. If not, follow what the other person said, contact administrators at 50BTC.com and ask them what has happened?
I would recommend contacting an administrator immediately. One thing is certain: they didn't just disappear into thin air. Most transactions take place under 24 hours, so it is highly unlikely that they will just show up in a week; however, don't be too disappointed about not getting them right away because they have been going up in value over the last few days