The key thing to note about reserved powers in the modern era, is that they are exercised on the advice and consent of the monarch's ministers. This is not just information to help the monarch reach a decision, but a binding requirement. From parliament.uk:
Government Ministers exercise the majority of the prerogative powers either in their own right or through the advice they provide to the Queen which she is bound constitutionally to follow.
The article goes on to describe how this is practically enforced through collective responsibility of ministers: a monarch cannot actually work effectively without ministers and civil servants, so on a basic level they need to be listened to for anything to get done.
This does not by itself mean the monarch can't take an active role in proceedings if necessary. But any actions would only be seen as legally valid if there was an argument to fall back on that they were being enacted on at least someone's advice, and with some elected minister's consent. For instance, if a highly unpopular bill made it through Parliament thanks to blatant corruption and there were armed rioters in the streets, the monarch would probably be able to provide a constitutional safety valve by refusing assent to the bill on the advice of a senior minister with some security responsibilities. This could likely be found legal later if the corrupt parliament was successfully ousted.
If the monarch decided to withhold assent because she doesn't like your face? Well, it would be analogous to a police officer arresting you because they don't like your face either. They might get away with the physical action in the heat of the moment - if they have a gun, it might be wise to comply - but (evidence aside) the arrest certainly wouldn't be held up as legal by anybody after the fact.
"Constitutional crisis" is often thrown around to describe such a situation. It wouldn't be the effect, however, but the necessary precondition - if Parliament's will is unified and recognized as legitimate, the monarch can't legitimately refuse assent (or take other such actions), because they wouldn't be acting in accordance with the requirements of the office. They would likely either be perceived as incompetent, or as attempting a coup (n.b. that some coups succeed, of course).
Since the situation is legally illegitimate, it doesn't need to have a specific plan beyond that for dealing with any other government officer who fails to carry out their duties, as adjusted to the officer in question. TL;DR: at the moment, no.