Any and all proof that the company existed, you worked there as whatever you claim to be, you were paid, and you performed the duties respective to your NOC. In addition to the letter from the employer, I would also add a letter of explanation saying that the business is no more and all the respective details with enough evidence and contact information of the former employer/supervisor etc. CIC gets such cases all the time and the more evidence you provide, the better it is for your application.
