Hi,
Welcome to the ehealthforum and I am glad to help you out.
There are many causes of male infertility and can be either pre testicular, testicular or post testicular causes. In pre testicular causes, it is due to low FSH hormone which is responsible for stimulation of testes to produce sperms. It can be due to hypopituitarism (for various causes), hyperprolactinemia and exogenous FSH suppression by testosterone. In testicular causes, the FSH is normal or high but the testes are abnormal, atrophic, or absent which are responsible for sperm production. Post testicular is due to obstructive azoospermia i.e. the sperms are produced normally but cannot be released in semen.
In your case, one testicle is there which can form sperms. You need to have a detailed evaluation from a fertility expert and get a thorough examination done.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.