As a leader , when you find out that one of your strategies for a long term plan failed , what should you do ?

Ans: Leader has his own conviction to pursue the goal through a strategy. When the strategy does not work and going to fail, the Leader analyze the environment and situation; vitalize and realign the strategy to achieve the desired goal. 

No plan survives contact with reality for long.   Long term plans always need to be adapted and shaped to circumstances.

Sometimes it is necessary to throw the whole plan out and do something else.   The whole point of having a leader is that they take responsibility for that decision.

Modify the plan, of course.

Most of the leaders will have multiple plans, if one fails they can imply other.

It is very important to monitor plans implemented and if required modify the strategies implemented when you feel it is going to fail.

all strategies are changed, you see, obstacles are expected in the strategy, you cannot have one plan that has no flexibility, it's the way business works, like chess, you start with an strategy, and change it to meet your ultimate objective.

In a typical well planned startegy this should not happen expect a rare event because the strategy would have all checks and balances at every level or stage of the program. If the event has indeed happened as you case one would always have a plan b or c in place and this would necessitate to activate the  other plans in the current scenario.

Simple, he will amend it as per the market trend whether it is long term or short. 

Good strategy and  plan must be set ahead in a manner  that takes in consideration risks and variation, therefore in case of any failure or amendment leader should be ready for alternatives; revise and modify or delete some steps...

It all depends on how leader is dealing with situation and the plan itself !

He/She must change the course instantly. Losing battle is not a prime but winning the war. Success has value when it has some account of failures.