Can you decrypt without key?
The answer to how to decrypt encrypted files without key is you can't. The only proven safe "encryption" is a one-time pad but that's very impractical... I'm going to save you the long, technical story. You have probably heard of some real world encryption algorithms: RSA, AES, RC4, etc.
How to decrypt text file?
Right-click on the encrypted file and select Properties. In the General tab, select Advanced. Now, uncheck the Encrypt contents to secure data radio box and click on OK.