Brief Answer:
Sebaceous cyst or lipoma are the commonest cause

Detailed Answer:
Hi.
Thanks for your query, albeit short and specific. Your mentioning your age, gender and details about the lump would have helped me more to assist you more - like:
Since when have you got the lump?
What is the size?
Is it slow growing over years or has appeared within days or weeks?
What is the color and condition of the skin above the lump?
Is it XXXXXXX or fixed?
Any additional information you would like to give...

The only information you have given is hard lump on the back of the neck.
The most probable causes of such a lump on the back of the neck are:
Hematoma due to old trauma or rupture of blood vessel.
Lipoma
Sebaceous cyst.
These being the mist common.
The lipoma will be painless and freely XXXXXXX the skin above it will be normal looking.The history will be of many months or years.
Sebaceous cyst may show a small spot with which it will be attached to the skin will be less XXXXXXX and not tender if not infected.
Other lumps coming from muscles will be hard and not XXXXXXX

I would advise you the following:
Get Examined by a General Surgeon and investigated like by ultrasonography if asked to and get it removed for 2 reasons:
- You will get rid of the lump
- You will get the confirmed diagnosis by histo-pathology examination.

I hope this answer helps you, please feel free to ask for further relevant queries if you need to or if you feel that there is a gap of communication.

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