Would I have a case if I worked in a building that had mold but the company claimed there was no mold, now a letter to employees there is mold.

Probably not.  Statute of Limitations in New York for personal injury cases is generally three (3) years from the date the cause of action accrued.  Although New York also has a "date of discovery" rule for exposure cases, such cases must be commenced from the date of discovery of the injury or from the date when the injury should have been discovered through the exercise of reasonable diligence, whichever is earlier.  Although you received a letter stating that the building was safe with no contaminants, you were arguably put on notice as far back as 2012 / 2013 that you had been exposed to mold (which started the proverbial clock).  Moreover, New York's courts have been pretty hostile towards mold cases with the State's highest court dismissing a mold case several years ago.