If my prostate biopsy comes back negative for cancer, what would be the next step in finding out why my PSA level was so high?

Send us PSA results. Without knowing how high is the actual value, we can not help. Send us the PSA result.
Depends on biopsy. depends on what is found. If biopsy is totally negative, might be sampling error and they just missed abnormal area. If it shows inflammation, you have your answer. Sometimes a prostatic massage and culture will prove prostatitis. How high is it?