Hello, and welcome.
I'm sorry you didn't receive an earlier answer and your patience is greatly appreciated!
If you lost your black briefcase during TSA security check-in, you can contact the TSA lost and found at the airport you were in. You didn't specify the airport here, so if you can tell me, I will be happy to look up the information for you, or you can do it yourself by going to the following webpage:
You can also file a claim for the lost briefcase, by following instructions here:
(Click 'File a Claim' up top)
As you can see, you can also email or phone them, here:
Email TSA Claims
I hope this helps and you are able to recover your briefcase.
Please be so kind as to rate my answer with positive feedback; that is the only way I receive credit for my assistance. Thank you very much!
Providing a positive rating will not end our conversation, should you need me for any follow-up. Simply click 'Reply' for clarification or additional information, if needed, BEFORE rating and I will be glad to continue at no additional cost.