It is important that you have your loved one's Social Security Card available especially when applying for state benefits.  If you don't know where the card is, start working on a replacement as soon as possible.  In the meantime, knowing your loved one's correct Social Security Number may be sufficient.  In my case, I never obtained a replacement card and that did not become an issue.  But, if you can and while you have time, get a copy of you don't have the original.

The Social Security Administration (see link below) has information online to help you with this task.