If the encryption method is an accepted standard method such as AES and the key is good it is not possible to find the key given the plain and encrypted data.
If the encryption method is trivial such as Caeser Cipher then it can be found.
If the key is trivial such as “password” or others found in lists of frequently used passwords it can be found with a brute force search.
solved How to decrypt Encrypted text to plaintext? [closed]