Can a person collect unemployment benefits in Illinois after long term disability through their employer and is able to work but lost their job?

The state you live in will not impact your eligibility to receive Disability benefits from a Long-Term disability policy. Therefore, Yes you are able to collect Long-Term Disability (LTD) benefits while receiving Social Security (SS) benefits.

To be more specific though, whether or not you actually receive benefits from your LTD policy will have everything to do with the Wording in the contract. If you have group coverage through work, then it is likely that any benefits you receive from Social Security will offset the benefits payable through your LTD plan dollar-for-dollar. In this situation, even though you are able to receive LTD from your company policy, you may not actually receive any from it if you are receiving Social Security benefits equivalent or higher than the monthly benefit it provides. The same consequences apply for Federal Government employees as through the FERS program. Additionally, if you own an individual policy that has Social Insurance offset benefits, then theses benefits will also be offset dollar-for-dollar.

If you own an individual LTD policy that does not have Social Insurance benefits than you would be able to collect on Social Security benefits and 100% of the benefits provided through your LTD policy.

This is actually one of the main reasons why individual LTD coverage is much better than group coverage.