They are suppose to give a refusal letter where you should be able to find the reason as well as UCI.
For UCI, if you don't have the letter and forget it, keep it blank. CIC themselve will give it for you.
For the rejection reson, if you don't have the letter, yes, you can write that. Also, you can add that your refusal letter has been lost so specefic reason cannot be given.
Generally the refusal letter contain reasons like 1. intention for not coming back 2. fail to show adequate fund 3.unemployed at origin country 4.misreprentation 5.travel history is not convincing enugh etc. etc.