I know hydrogen is a non metal, but when I just study about some introductory elementary band theory I find the band structure of hydrogen has a half filled valence band just like alkali metals, and this should make it conductive. Is there some thing more complex about it or is my understanding is wrong? How can the non metallic nature of hydrogen be explained using band theory?
Solid hydrogen is not made up from hydrogen atoms. It contains hydrogen molecules held together by relatively weak Van der Waals forces. In the hydrogen molecule the two $1s$ levels of the atoms form a bonding $\sigma$ molecular orbital that is full and an antibonding $\sigma^*$ orbital that is empty:
In the solid the $\sigma$ level broadens to form a full valence band and the $\sigma^*$ level broadens to form an empty conduction band, with a gap of around 15eV between the two bands. That is why solid hydrogen is an insulator.
In solid hydrogen the electrons are localised in H2 bonds. There are various crystal structures. The band structure of course depends on this. Upon very strong compression the crystal structure should change to a high coordination characteristic of a metal. The H2 bonds then disappear. So the behaviour is more complex than just a shift of bands. It is a structural and electronic phase transition. Search for metal-insulator transition.