April 15, 2020
College officials really don't want to rescind acceptances, especially at this late date. Just as you're making plans for move-in and purchases for your dorm room, they, too, are finalizing details and databases and don't welcome the disruption that losing an enrolled student would provide.
In a perfect world, however, you would have written to the college folks right when high school ended (or even sooner) to warn them of your drop in grades and to show that you were aware of this issue and that you cared about it. Ideally, too, you would have been able to offer a reason for your downturn such as a protracted illness, a death in the family, problems on the home front, etc. But now, the best you can do is to write back immediately and grovel to retain your acceptance. If you can't cite any of the aforementioned excuses, then at least be very apologetic and explain just what you've said here ... that you were burned out. Offer assurances that this summer has given you time to recharge your battery and that you're eager to return to the classroom. Add specific examples if you can (e.g., “Working at a camp for children with autism has fueled my interest in a career in special education" or “After two months of flipping burgers, late nights with a chemistry text actually sound exciting!")
You can also put a deal on the table by saying something like this: “I want you to have faith in my intention to be successful at college. If you allow me to matriculate this fall, I can meet weekly with my advisor (or a dean) to prove that I'm staying on track."
You don't tell “The Dean" just how badly you stumbled. When you say that your grades were “so much lower" senior year, did you slip from a 3.9 GPA to a 3.0 GPA or to a 2.0? If the slide was really severe, the college may decide to make an example of you and revoke your admission. But if you managed some good grades along with the lousy ones, you'll probably be okay. In any case, you need to respond now and do your best to convince admission officials that you're ready to buckle down.
On the other hand, maybe you're NOT all that ready. If you're still feeling burned out, ask instead if you can defer your acceptance for a year. Many students find that a gap year is a great way to take a break from the demands of school and to then return with renewed enthusiasm. So perhaps the letter you just received is a blessing in disguise, and you should take time off from academia. If you do, the items you've already bought for your dorm room will be waiting when you finally need them.