Well,It's impossible, lets say for me, to write the complete answer to the question. However I can give you some hints.

You may start the answer by giving a brief account of the race for colonialism/imperialism in Europe(15th century onwards). After that you might want to name the colonial powers that wanted to pocket India for its riches, i.e. Portuguese, Dutch, French and last but not the least Britain. You would also need to write the conditions/factors that favoured them to achieve their ends. And to the extent that I know, these were the same as the ones that caused demise of the islamic raj i.e

1. Weak Successors of Aurangzeb
2. Vastness of the Empire
3. Absence of a Definite Law of Succession
4. Deterioration of Morality of Mughal Emperors
5. Degeneration of Nobility
6. Deterioration of Army
7. Intellectual Bankruptcy
8. Mughal Rule Alien, to Indian Soil
9. Corrupt Administration
10. Stoppage of Adventures from Persia, Afghanistan and Turkistan
11. Invasions of Nadir Shah and Ahmed Shah Abdali
12. Aurangzeb's Policy in Deccan
13. Absence of Naval Power
14. Plight of the Common People and Peasants
15. Rise of Marathas
16. Rise of Sikhs
17. Rise of Hindus of Rajputana
18. Backwardness in Science and Technology

Hope it helps. And I hope that I'm not wrong.