By select an item from the dropdown list, the other field(text field) will be auto populate from the database and locked the field

You need to write a server side dropdown selection index changed event or do a jQuery Ajax call onchange of dropdown to service or something which will retrieve the data from database and return.

Then just assign the return data into the text box. I hope locking means disabling the text box. That you can easily do by making it read-only.