If by "help" you mean modern psychatric therapy, then no, that wasn't around before the 1800s. Mental illness, however, was around and the affected had to manage in other ways. Similar to the history of science, our main problem when talking about the history of medicine and healthcare is that we think in categories heavily defined by the last 200 years. "Patient", "therapy" and even "mental illness" are concepts that don't translate very well to earlier times. Without the clinical distinctions made today, a wide spectrum of abnormal behaviour was grouped together, from sadness to madness. Linking that notion of abnormality with our concept of "mental illness" is difficult at best. Nevertheless, it's the only word I have at my disposal.

Comprehending the steps the affected could take is equally difficult because it requires some psychologic understanding. A great read on the history of mental healthcare is "Madness, Religion and the State in Early Modern Europe: A Bavarian Beacon" by David Lederer. The book only covers the time after 1400, but it is quite successful in giving a rough grasp of the mentality of that time: Since religion was a fundamental part of life, the cause as well as the treatment of mental ailments was often connected to the spiritual wellbeing of the patient. As such, people frequently turned to the church to be cured - more so than with "conventional" illness. Among others, Lederer describes how the common practice of making a pilgrimage to the church/chapel of a specific saint associated with a disease in order to be cured was also seen as a remedy against mental problems. In cases were the patient was unfit to travel for whatever reason, it was apparently not uncommon for a family member to undertake the pilgrimage instead, so a mental patient would not have had to be clear-headed enough to understand their disease. All this might sound strange at best and superstitious at worst from our modern viewpoint, but it did provide some relief. Especially with mental illness, putting the patient's mind to rest can go a long way towards recovery.

It's impossible for me to comment on to what extent mentally ill people were killed. But since there were probably fewer children born mentally disabled who made it through to adulthood, I assume that mental illness was seen more as a passing malady that the affected might be saved from eventually. Since there were steps to be taken against it, it does not seem as if there was a large stigma attached to mental illness, at least speaking for that time and place (Early Modern Bavaria). I'm sorry I can't supply you with information specifically about the middle ages, but I hope I could give you at least some insight.