I got an address and received a private key on the email and when I looked at the blockchain one week ago, I saw there ony one transaction of $ 300 .
Which wallet were u using?
Sending/recieving private keys via email is a big big BIG mistake.
Email is unencrypted. Additionally emails can be forged, intercepted, ... you name it.
Email is an outdated protocol (from the 1980's). Just because its used everywhere, it doesnt mean its safe.
Of course its not that unsecured if you were encrypting your mails with PGP.. but im assuming you didn't.
When I looked at my blockchain address yesterday, I saw there two transaction and my address has now 0 bitcoin.
Could anyone please tell me if someone stole my bitcooin or what happened
If you didn't initiate the second transaction.. then yes. Unfortunately it seems your BTC's have been stolen.
And if I still have my private key, can I ask for my bitcoons back?
Unfortunately not.
Your private key is associated to the address.
With a private key of address A you can only spend from this address A. Everyone with access to a private key can sign transactions (send btc's) from the corresponding address.
For the next time i would suggest to make sure your PC is clean from any malware.
Its not certainly because your mail was spyed on. It may be 'just' an infected PC.
To be absolutely sure you should wipe your HD (after backing important files up) and start with a fresh installed OS.
And
never ever recieve/send private keys via mail!