A stiff neck could be caused by any of several factors.
Some things that come to mind:
Vision ~ have you had yours checked recently? Make an appointment if the answer is “no,” even if you don’t think it’s necessary.
Do your glasses need to be adjusted? If the frames are out of whack, your vision will be affected.
If you wear contacts, do you need a new pair? Or are you making the weekly plan last for a month?
Dental problems may be causing tension.
Body alignment ~ head over shoulders, not forward
Feet firmly planted flat on floor to support the upper body
Does the printed music need to be closer? Farther away?
Are you seated too close to the piano? Too far away?
Is there a draft?
Are you seated with your back next to a cold window?
Is the room warm enough? Muscles become tense in cooler temperatures.
Lighting should illuminate the music and the keyboard evenly, without shadows. Is the light bright enough?
Eliminate glare ~ no plastic sleeves on music, no glare on glasses
Light should not be shining into your eyes.
Tension in shoulders or jaw creeps to neck
Do you breathe when you play?
Do you take breaks? You need a break once every hour.
Do you stretch before practicing?
Do you warm up?
Is the music easy to read? You may want to make enlargements.
How many pages are set up at one time? Four across may be too many to read comfortably.
Is the score directly in front of you?
Do you vary the level of difficulty in your practicing? When there is a deadline approaching, such as a jury, audition, or recital, we often tend to trade awareness of good habits for more speed and volume. (“I’m gonna get this right!”) Playing fast and loud for long periods can exacerbate tension.