The Prophet became sick and did not get up (for Tahajjud prayer) for a night or two.
Narrated Jundab bin ‘Abdullah :
Gabriel did not come to the Prophet (for some time) and so one of the Quraish women said, “His Satan has deserted him.” So came the Divine Revelation: “By the forenoon And by the night When it is still! Your Lord (O Muhammad) has neither Forsaken you Nor hated you.” (93.1-3)
Tahajjud was, as we all know, compulsory for the Prophet (SAW). He used to offer it every night. But when he became sick, he did not (maybe could not) get up for it. He was a human. So are we. It happens sometimes that we miss out on our regular, habitual deeds due to sickness or traveling or any other exceptional situation. But look at the immense mercy of Allah. He still rewards us for those missed out deeds. :)
Prophet (SAW) said:
إِذَا مَرِضَ الْعَبْدُ أَوْ سَافَرَ، كُتِبَ لَهُ مِثْلُ مَا كَانَ يَعْمَلُ مُقِيمًا صَحِيحًا
When a slave falls ill or travels, then he will get reward similar to that he gets for good deeds practiced at home when in good health. [Bukhari]