There are several things you must do in order to successfully sue over contaminated food or drink.
First, you need to seek medical help soon after you become ill. You will need to have a stool sample and a urine sample for testing, and you will need to have medical reports, to support your contention that what you are suffering from medically is connected to bleach consumption (or some other contaminant).
Second, you need proof that the drink was, in fact, contaminated and you will need evidence of the contaminant.
Third, you will need to prove that the coffee was contaminated at Dunkin' Doughnuts. When and where the bleach was added to the coffee is likely to be challenged.
Once you have good evidence that your symptoms or illness or injury resulted from drinking bleach-laced coffee from Dunkin' Doughnuts, you will need to prove that Dunkin' Doughnuts was negligent - that Dunkin' Doughnuts failed to exercise reasonable care when making and serving their coffee.
A lawsuit can potentially seek compensation for significant medical bills, rehabilitation costs, lost work time, lost income, out-of-pocket expenses (and, not that this applies to you, funeral and burial costs).
I suspect that you will have a difficult time proving all of this well enough to support a lawsuit. But facts matter. Perhaps you can use the "reply to thread" feature and add more details (as your post seems devoid of them, probably as a result of the malfunctioning "preview post" feature on this site which deletes posts upon submission).