Travel insurance is designed to protect against unforeseen losses which could potentially happen in the future, not to protect against losses which have already happened. You are only covered for losses which occur “after” the policy effective date.
In addition, travel insurance usually covers trip cancellation for specific perils or events as listed in the policy (i.e. injury, illness or death, etc.) For reasons not listed in the policy, you would need a policy with a “cancel for any reason” option.