She cannot move her head because of severe pain; how should she pray?

Praise be to Allah.

Firstly: 

We ask Allaah to grant our sister healing and well being in her religious and worldly affairs. 

Secondly: 

If the time for prayer comes and she feels pain, then she may delay the prayer until the end of its preferred time, in the hope that she may be able to offer the prayer in the proper manner. But if the pain persists, then she may pray in the manner that she is able to, even if she is lying on her back. 

The basic principle concerning that is the verse in which Allaah says (interpretation of the meaning):  

“Allaah burdens not a person beyond his scope”

[al-Baqarah 2:286]. 

And the Prophet (peace and blessings of Allaah be upon him) said to ‘Imraan ibn Husayn: “Pray standing, and if you cannot, then sitting, and if you cannot, then (lying) on your side.” Narrated by al-Bukhaari (1117) and Abu Dawood (952). 

If a person prays sitting, then he should lean forward for rukoo’ (bowing) and sujood (prostration), and he should lean forward more for prostration than for bowing. 

If a person prays lying on his right or left side, then he should tilt his head towards his chest for bowing and prostration.  If a person prays lying on his back, he should put his feet towards the qiblah and tilt his head towards his chest for bowing and prostration. If he is unable – when sitting or lying – to move his head, then the actions are waived in his case, because that is the thing that he is unable to do, but the words are not waived because he is able to say them, and Allaah says (interpretation of the meaning): “So keep your duty to Allaah and fear Him as much as you can” [al-Taghaabun 64:16]. So he should say takbeer and recite Qur’aan, and intend to bow and say takbeer and recite the tasbeeh of bowing, then he should intend to stand up and say Sami’a Allaahu liman hamidah, rabbana wa laka’l-hamd (Allaah hears the one who praises Him, our Lord to You be praise) etc. Then he should intend to prostrate and say takbeer and the tasbeeh of prostration, because this is what is implied by the shar’i principle, “So keep your duty to Allaah and fear Him as much as you can”. 

End quote from al-Sharh al-Mumti’ (4/332). 

And Allaah knows best.