Hii Harsh,
I would like to help you with the same. I will talk about general method to retrieve both your User Id and Password which will work for almost all application portal since you haven't mentioned any specific application portal.

1. Check your messages and Email id (do check spam folder as well). Many a times, you can get your login credentials from there. If password isn't mentioned then just take the User Id, go to the application login portal and get new password by clicking on Forgot Password.
2. If you have use Chrome Browser to access the page, then Chrome Browser also saves your Login Id Credentials, provided you have given access to it. Do check you Chrome data and browsing history, it may help you.
3. In case you can't retrieve both the things, then drop your query to the official mail id of the concerned department and write your concern over there. Also, do write your basic information like Name, Father's Name, Mother's Name, Phone no. , Address, Application number(already mentioned) that you have used while filling the application. It may help them to search your query from their database.

Feel free to ask me any further doubts.