Yes, there is "cannot" and "cannot". The other "cannot" actually means "shouldn't under any circumstances whatsoever".
By doing this you're violating the Thread class contract. Using reflection allows you to do a lot of things that designers of the Java API didn't foresee and can have unintended consequences. It is possible that a new thread was not actually started, or if it did start, it might operate incorrectly. ThreadLocal variables would quite certainly be messed up in that case, for example. And I would not be surprised if the JVM crashed at some point after doing this.
Why bothering starting the thread again? Creating a new instance is way easier, and infinitely safer.
By the way, using bold on your whole post makes it harder to read. Any emphasis or highlight is best used just on a few words you want to get more attention to. Anything else is quite counter-productive.