In the Reports tab, create a new report using "Leads / Leads with converted Lead information" if you want to get account, contact, and opportunity information side-by-side with the lead, or you can use a normal "Leads / Leads" report to get any fields from the lead that have been converted. You can add a filter for "Converted equals true" to find just leads that have been converted. Viewing a converted lead directly simply shows the account, contact, and opportunity (if any) it was converted into.
If you don't have access to this information, it's possible that your profile may be missing some permissions; contact your administrator for more details. If you are an administrator, then contact support for additional assistance.