WARNING 000117: Warning empty output generated. Executed (Buffer) successfully.
If that was the error message, perhaps you were doing a negative buffer and it was too large a value resulting in null geometry...which is what your output suggests.
If that wasn't it, then you need to provide details. And I am assuming that you are buffering projected data using a buffer size in the same units and the inputs and outputs are different...plus anything else I have forgotten to mention