Calm down.

The proofs of previous employment are needed for LC. And the whole packagen will forward to I140 process once it's approved.

What does your lawyer say about this ? I think he/she is the first one you should talk to. Why does one of your previous employers refuse to give you a proof of employment ? This is something you need to focus on. I am not sure who did you talk to, the HR or your previous manager ? Just keep working on that. If you still cannot get anything, here are something you might want to look into:

1) What's your status on that job ? OPT, H1B, or .. ?
2) If it's H1B you should have I797 for that. After you change your job again, you should have your H1B transfered to your new employer, meaning a new I797. So, the I797 should state clearly who you work for and the time difference should be the time you worked for that employer.
3) Dig out your tax return and paystubs to support your point.

The proof of employment not only shows your experience in the job you do, but also shows you were employed at that time to match with your immigration status. However, it will still be odd if you cannot get a letter from your previous employer with only a sentence on it. Talk to your lawyer and keep communicating with your previous employer. Goog Luck.