If you are eligible for Medicare because of a disability, ESRD or ALS, you are eligible for premium-free Part A.
If you return to work, as long as you remain medically disabled you can keep your Medicare coverage indefinitely. For the first 8.5 years you will not have to pay the Part A premium. After that, you will have to pay the Part A premium yourself. If at that point your income is too low to afford the Part A premium, there is a program called the Qualified Disabled and Working Individuals (QDWI) program that can help.
To find out how to continue to receive Premium free Part A if you have a disability but lose Social Security Disability because you return to work, click on the FAQ.