King Henry is a gracious lord. He takes an obvious interest in your cause. You swear fealty to him for Normandy, and he promises to escort you himself to Rouen and stand by as you summon your magnates. In a few months, this transpires according to plan. The Norman barons are cowed by the prestige of the king of France, and appear to come to heel. In his presence, they one by one renew their fealty to you. Then after a feasting time, king Henry bids you adieu.
For a brief while, you appear to be in command of Normandy. But really, outside of the environs of Rouen itself, you have little control and no forthcoming resources to speak of. When a baron is reported to you as guilty of making war on other of your vassals, you have no sufficient power to prevent the violence to your realm's peace. You forbid the construction of castles but nobody listens. Your realm is dissolving rapidly into anarchy, and you are powerless to prevent it. Even in Rouen people are going about looking as though they feel that you are not doing what you ought to remedy the situation.
Then your enemies cooperate enough to get an army together. You have a few choices in how to deal with this new, unprecedented threat.
Flee to king Henry and beg his help.
Stand a siege in Rouen; it is a formidable fortress, after all.
You can take your household knights and servants and sail to England, to recruit soldiers from your cousin king Edward.
You can stand a siege in Rouen and send for aid to king Edward.