The decline of the Sikh Empire can be attributed to the following factors:

• The death of Maharaja Ranjit Singh led to internal corruption and turmoil
• The quality of the army declined due to mismanagement
• The two Anglo-Sikh Wars saw the defeat of the Sikh Empire on both the occasions. Ultimately it was dissolved in 1849.

For further reading check the following articles:

• First Anglo-Sikh War
• Second Anglo-Sikh War
• British Raj