Failed Background Verification - was working in a big company on the payroll of a consultancy and that consultancy is blacklisted

Hi, I am currently working as a Software Developer in a very reputed company. It has been 4 months since I have joined. And suddenly yesterday HR contacted me that my Background Verification was failed because of my previous company. Previously I was working in a big company on the payroll of a consultancy. Now the problem is that consultancy is blacklisted and that's why BV has failed. Now I have been given 5 days to prove that I was really working in that company. I have lost faith in the current company as in these 4 months they could not reach out to that company even if the consultancy was blacklisted. What should I do?

IT is the duty of the HR to get in touch with the consultancy and verify. Just because consultancy is blacklisted doesnt mean that the employee did not work. Background verification of candidate experience is to actually ascertain whether you really worked there or not and is the certificate really issued by the company / consultancy and is not a fraud one.
The HR of your company seems incompetent fellow in your case. Such HR who half half knowledge are a virus to the whole of HR community.

HR needs to take more pains in proper verification.
Is the consultancy still in existence?
Were you on the employee strength of the consultancy or the reputed company??
Probably you can speak to the consultancy and request them to speak to your current HR.
You can facilitate this interaction to sort out the problem.

Yes, you must contact your last company or any designated person. Every problem must have a solution. Else, talk to your immediate supervisor .

I was full time employee for my consultancy and was outsourced to another company. Now they have asked me to submit the details of that company where I was outsourced. Can I file a legal complaint in this case if I loose my job because all of this?

What all documents do you have proving your work with the Consultancy?
Do not think that you will loose your job-be proactive .
Get back to the consultancy(previous employer) and ask them to contact your HR.
Your HR seems to be taking task of verification little bt easy.
They should be aware that lazy verification can cause inconvenience to employee being checked out.

your original employer (consultancy) is blacklisted - by whom?
even if it is blacklisted - why is it not giving response to a background check? is the consultancy closed altogether?
the large company, where your were deputed by the blacklisted consultancy company, can perhaps give a certificate that you worked on deputation from the consultancy from..... to ....... why not try that ? and the consultancy company would have given you deputation letter, salary slips, appointment letter, joining letter, increment letter (if any), leave approvals .... talk to the present employer and see if the good sense prevails.
A S Bhat