Why it showing error in modifying db in MVC in edit action

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SQL-Server

LINQ

MVC4

MVC5

I have a confution in my project that when I am modifying the data to db it shows the error that "

Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=472540 for information on understanding and handling optimistic concurrency exceptions."

I am getting the value which I needed in the HttpPost of edit action method, the error arise only when saving it to the db
controller

[HttpPost]
        [ValidateAntiForgeryToken]
        public async Task<ActionResult> Edit( StudentDetail studentDetail )
        {
            if (ModelState.IsValid)
            {
                studentDetail .CashInHand -= studentDetail .CashReceived;
                studentDetail .Amount += studentDetail .CashReceived;
                studentDetail .Date = DateTime.Now;
                studentDetail .CashReceived= null;

                db.Entry(studentDetail ).State = EntityState.Modified;
                await db.SaveChangesAsync();
                return RedirectToAction("Index");
            }
            return View(studentDetail );
        }

I am getting details in studentDetail. Then Why is that the error showing ???can anyone please help me to find the solution ??

What I have tried:

I have search the my error but i didn't understand why is that happening Can anyone please help me to find the solution ??