Why does a locked mutex prevent http.ResponseWriter from sending a response?

Question

I understand that one should unlock a mutex using defer in case an error comes up. Otherwise, if you left the mutex locked, other goroutines would be blocked if they tried to access it. But what I don't understand is if I was to remove or comment out the h.db.Mutex.Unlock(), then there will be no http response sent back to the client. Again, I understand leaving a mutex locked would block other goroutines from aquiring the lock and it would be very silly to not unlock a mutex, but why would it block the goroutine that FIRST acquired the lock from being able to send an http response?

type HandlerWrapper struct {
    db *sql.DB
    dbMutex *sync.Mutex
}

handlerWrapper := HandlerWrapper{db : db, dbMutex: &sync.Mutex{}}

// using go chi router
router.Get("/items", handlerWrapper.GetAllHandler)

http.ListenAndServe(":5000", router)

func (h *HandlerWrapper) GetAllHandler(w http.ResponseWriter, r *http.Request){

    h.dbMutex.Lock()

    //defer h.dbMutex.Unlock() // if this is commented, no http response, why?

    rows, err := h.db.Query("SELECT * FROM lst") // this line returns an error intentionally
    if err != nil {
        log.Printf(": %v", err)
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }