There is a tree forum here that may be of more help but in my opinion I don't think such a mature tree could have suffered so quickly from having its bole buried. However if this landscaper is a complete clod (and knowing not to do this is very basic) it is possible that he did further damage to roots and bark.

If the soil level was changed to that degree, then yes, the tree has likely suffered profound damage. Even as little as 2-3 inches of increased soil grade can permanently damage or kill a tree. Often, the damage will not be apparent immediately - it can take several years for the impact to be clearly seen - but depending on circumstance, it could be much faster.

The only way you will know for sure is to have the tree assessed by a qualified arborist. But it takes virtually no time to smother the fine feeder roots that supply the tree with oxygen, soil nutrients and moisture. If this happened last year, then it is already too late to make any corrections. Best to prepare yourself that the tree may very well have to be removed.

As Ink stated - this is pretty basic knowledge for anyone familiar with horticulture or an experienced landscaper. You DO NOT CHANGE the soil level around established trees. Even lowering it can impact tree health but raising it to any significant degree is the kiss of death.

Thank you so much everyone. I repsoted in the trees forum...thanks for advsiing.
And also thank you very much to all that assisted thus far. Much appreciated everyone.

We will proceed with undigging as far as we are able to atleast try and save the tree. Might aswell do as much as possible and hopefully it will heal and come back to life.

Thanks everyone.