There are two main types of hearing loss:
1) Conductive hearing loss where the transmission of sound to the inner ear is impaired by a problem either in the external ear canal, or the middle ear space. Examples of this type of hearing loss include infection in the outer ear, impacted ear wax, perforation of the tympanic membrane, glue ear and disease in the middle ear space causing fixation or erosion of the ossicles (bones of conduction).
2) Sensorineural hearing loss where the sensitive hair cells inside the cochlear (organ of hearing) or the auditory nerve are damaged either through aging, disease or trauma. Examples of this kind of hearing loss include head injury, the side effects of certain drugs (eg certain ototoxic antibiotics and cancer medication) and presbyacusis (the normal deterioration of hearing with age).
Obviously precise diagnosis of your condition will depend upon taking a complete history, performing a detailed examination and arranging a number of specialist tests.
It will be important to determine whether you have suffered any recent trauma, whether there are associated aural symptoms such as dizziness or tinnitus, whether you enjoy good general health and obviously family history and any medication you may be taking are also important. Similarly a full otoneurological examination will be required. Specialist tests may include audiological investigation, blood tests, for example looking for infection, and occasionally radiological examination. It is generally accepted nowadays that any patient presenting with unilateral tinnitus, hearing loss or vertigo for which no explanation can be found, should undergo an MRI scan to exclude the presence of an acoustic neuroma, which is a benign growth on the nerve.