In completing the 5500, I realized that a plan should have been filing a Schedule A... however, it has been missed since 2000. It was filed in 1999 but we haven't received any DOL letters/requests for it yet. Do I go back and amend 2000 and 2001? I have never received any information from the insurance company (and the info I did receive this year isn't complete- they only provide Part I data). Will they be able to come up with the past data? Or is there an entry I can put in that they did not provide it?

Advice or suggestions?

ERISA requires the insurance company to provide the data to the plan administrator, but it requires the administrator to file a complete an accurate Schedule A, regardless of whether the insurance company provided the data.
That means you have to try to get the data from the insirance company, and if you can't, you file A with the best data you have, estimate if you have to, and feel free to indicate if the insurance company is not cooperating.

Regarding prior years, the technically correct answer would be to amend. Failing that, at least gather the data you need and be ready to prepare Schedule A's, because if the IRS/DOL ask for the data, you will not have much time to comply.

"Or is there an entry I can put in that they did not provide it?" Yes - see the Schedule A instructions about what to put if you have requested the info and the insurance company will not provide it.