Ringing, or any sounds from your ears/brain, is referred to medically as tinnitus. It is often the reason that someone may pursue hearing testing. However, what most people desire to know is the reason for the noises in their ears. Although, no one can answer that question definitively, there are a few theories out there.
Hearing loss: 80% of people with hearing loss report ringing in there ears. 80% of people with ringing in their ears have hearing loss. In the audiology world, this is referred to as the 80/80 rule. The most common reason I read and hear about is a theory that the brain is trying to replace sound that is no longer there. This seems plausible, but it doesn't account for the 20% of people with hearing loss that don't report tinnitus OR the 20% of the people that have tinnitus and no hearing loss.
Wax (cerumen) impaction: I worked in the medical field in various settings for 25+ years. When a patient visited the office because of incessant ringing in the ears, we always checked for a plug of wax in the ears. Once the wax was removed, they would be thrilled that their tinnitus had disappeared! Now, we do know that wax DOES NOT cause tinnitus; however, it can exacerbate the ringing. In other words, the tinnitus is not loud enough for the patient to notice until wax plugs the ears enough to block out external sounds so that the ringing becomes more obvious.
Noise exposure: Have you ever experienced your ears ringing after attending a loud concert or participating in another loud activity (gunfire or firecrackers to name a few)? Often that ringing will last a day, or so, then disappear. What we believe is happening is that the hair cells (nerve endings) in the ears bend (but don't break) causing temporary symptoms (hearing loss and ringing). They can recover and the symptoms go away. However, if you continue to experience these symptoms for more than 72 hours after the loud noise exposure, there is a good chance that the symptoms will be permanent.
Other health conditions: There is also a correlation between certain health conditions and tinnitus. To name a few, people with high blood pressure, high cholesterol, TMJ (temporomandibular joint disease), and head and neck injuries, often report ringing in their ears (without an accompanying hearing loss). Most people think this is a result of change of irregular blood flow to the ear.
Pharmaceuticals: There are a number of medications that are oto-toxic. Meaning they are toxic to your ears. This includes some antibiotics, anti-cancer drugs, and high doses of aspirin, to name a few. If your ears start ringing after starting a new medication, call your pharmacist to see if the medication is oto-toxic.
This Healthline article does a good job summarizing the causes and treatments of tinnitus:
Finally, if you are experiencing continuous ringing in your ears, schedule a hearing test with an experienced audiologist!