Q: After which war Mughal Empire was re-established in India?  

After the death of Islam Shah Suri, the Suri Empire was in a civil war where various contenders to the throne fought each other for supremacy. Sikandar Shah Suri was occupied with his struggle against Ibrahim Shah Suri when Humayun mobilized an army from Kabul. He captured Rohtas Fort and Lahore in February 1555. Another detachment of his forces captured Dipalpur and Jalandhar. Their advanced division proceeded towards Sirhind. Sikandar sent a force of 30,000 to intercept them but they were defeated by the Mughal army in Battle of Machhiwara and Sirhind was occupied by the Mughals.

Sikandar led an army of 80,000 and met the Mughals at Sirhind. On June 22, 1555, they met in battle. Humayun and Bairam Khan carried out a daring raid in a rainstorm like Sher Shah Suri had done at the Battle of Chausa to defeat Humayun. Sikandar was defeated by the Mughal army and was compelled to retreat to the Sivalik Hills in northern Punjab. The victorious Mughals marched to Delhi and occupied it and reestablished their empire in India.