When a lead is converted successfully in Salesforce, you would noticed that you could not access the lead information directly even though you recorded its ID. Example:- https://ap1.salesforce.com/00Q9000000DOG9g
Using the ID method to access the converted lead information would redirect you to a different page than the usual lead record page. i.e.
Rest assured that this does not indicate that the information/lead record is forever deleted from Salesforce. One way to retrieve the converted lead information is through Lead reports.
Alternatively, you could retrieve the converted lead information via API such as exporting the information through a dataloader. The following illustrates CSV content exported from dataloader for a lead object:-