Hello, happy to help.

If your arborist thought the trunk was girdled and didn't offer any remedies at the time, it's pretty likely it was not in a state that could benefit from remedial treatment. That involves digging down to the root flare then cutting away roots that are encircling the trunk above the flare. I won't go into the further details of that but a tree can take that as long as 20 years before it succumbs.

Maybe over the years, you haven't noticed some gradual thinning of the canopy so that it may have appeared to look just fine to you. And this spring, it is either taking its time to leaf out (most lindens have been leafing out for more than a month by now) or has simply died.

At this point, I'd recommend having your tree evaluated again by a certified arborist to determine how hazardous it may be even if some leaves are actually starting to appear. If the tree is close enough to the house or over a sidewalk or street, it's important to ensure it doesn't hurt anyone or anything in a big wind or storm.

Here is a link to information about how to find and hire a professional.

Good-luck!