Hi!
You should take rest and have some analgesic/pain killers and apply hot water bag to neck and areas of pain initially if it's too severe otherwise just go to a good physiotherapist to check from where actually pain is coming and take treatment there. Stop head down activities and I think you should not exercise while in pain. Once pain subsides u can definitely do neck and shoulder strengthening exercises. Don't forget to drink plenty of water!!
Next Steps
Consult a physiotherapist or an orthopaedic doctor
Health Tips
If you have more work on computers,google computer ergonomic advices or ask to physiotherapist near u and follow it!