Actually, this is not possible,
Once in the hypnotic state, we are essentially in a very similar state to that of nighttime sleep. If for some reason the therapist left the room, you would eventually come out of the trance state, usually in minutes. This is the same response you have on a daily basis when you awake from sleep. Perhaps one major difference, specific to hypnosis, is that relating to the production of melatonin? So, although you may not realise it; you are most likely very proficient in accessing states akin to hypnosis.
While you will not get stuck in hypnosis, you are often more likely to have an issue getting into the state than remaining in it. Some of the inhibitors of entering hypnosis, in the clinical environment, are things like apprehension, anxiety, scepticism and/or doubt. These can interfere with the client's ability to effectively access these specific sleep states. This is simply because all of the aforementioned are more consciously driven aspects of being human! However, as an experienced hypnotist, I will recognise this and help to guide you by finding effective ways to access these deeper states of mind!
Some people may see this as a form of manipulation but it really isn't! After all, why would you go to see a hypnotherapist if you did not want or expect to be hypnotised? In this sense, the hypnotist is merely facilitating your intended purpose in being there? It is also worth noting, that when working with an experienced hypnotherapist, the resultant trance often produces a very profound state of deep relaxation. Perhaps deeper than you have ever experienced in your life before? Consequently, it is very common for clients to say "I didn't want to come back!" This is why I allow the client to regain their sense of normal awareness in their own time; naturally! Some hypnotherapists erroneously describe hypnosis as a deep state of relaxation. This is incorrect because relaxation is merely a collateral effect of hypnosis, not what it is!