The initial signs of depression can be simple things such as changes in eating habits or sleeping patterns. Low moods leading to periods of feelings of despair often follow. You may become emotionally detached from loved ones, friends or relatives which leads to a feeling of isolation.
Extreme or prolonged depressive feelings can lead to overwhelming thoughts of self harm or even suicide. This doesn't mean necessarily that you are likely to carry these things out but obviously these are major warning signs.
There is no one cause of depression as genetics play there part as much as external events do so it is important to discover the whole picture to enable you to move forward.
One thing for sure...depression is an illness that can be treated...there can be an end to this for you.