Nobody actually knows what Elizabeth I died of but it could actually be simple old age as she was 69, a great age in Tudor times. There was no post mortem, so all we can do is theorise, but it may be that she died of blood poisoning after decades of using ceruse, the white face make-up she was so famous for, which contained lead, but it may also have been cancer, we just don't know. She did seem to sink into a depression after the execution of her favourite, Robert Devereux, Earl of Essex, and the deaths of close friends, so perhaps she also lost the will to live.
There was an interesting cause of death given on the Internet a few years ago, which I can no longer find that said she had a specific illness caused by an infected tooth. (Her teeth were terribly decayed and black.) The condition would result in a root canal today, but that wasn’t done back then.
The disease has a name, but I don’t recall it. In any case, the infection from the tooth spreads down the throat and neck, infecting the glands there and eventually shutting off her bronchial tubes, causing death. This disease fits all her last symptoms, sucking a finger, refusing food and drink, not being able to speak. The article was quite convincing.
Elizabeth could have died from a combination of things. While her lead makeup posed a risk to her health, you have to remember that she was getting up in age. Also, London and England in general was not the most healthiest place to live during the time. People did not dispose of their garbage properly and at times, English cities could become very filthy. Furthermore, you could look at other members of the Tudor family to help you determine Elizabeth’s death. For example, it is believed that Henry VIII died of untreated type II diabetes, and that Mary I died of uterine cancer. Both of these diseases can be hereditary, so maybe she unknowingly suffered from one of them. Like the previous comment stated, Elizabeth did suffer from oral infection(s). In recent years, it has been discovered that certain oral infections can cause diabetes and other illnesses.