Inactive TS/SCI CI poly

Can someone tell me is there any way to have a security clearance re-activated if it has not been used for more than three years?

Also is there a way to verify that I had an FS poly clearance years ago
and that I was investigated for a Yankee white clearance?

You probably can’t have it reactivated, almost certainly have to start from scratch. However, I have heard of some folks who got lucky.

There’s probably a record of that poly somewhere; whether or not it can be easily accessed by whoever might sponsor your new clearance is another question. No idea about the other thing.