Contact the casino support and ask about your options in this situation. Some online casinos will allow you to make a withdrawal if you agree to forfeit the bonus money and any winnings pertaining to that bonus money.
Also ask the support about how much more wagering you must to do fulfill the wager requirement. In some situations, completing the wager requirement will be a better choice than forfeiting your bonus and winnings. In other situations, the wager requirement is so ridiculously large that relinquishing the bonus and any winnings is the best course of action.
Sometimes, knowing how to calculate the wager requirement is difficult and can cause a player to believe that he has fulfilled the wager requirement when he actually has not. For instance, it is not unusual for a deposit bonus offer to include some very fine print that states that the wager requirement is calculated based on both the deposited amount and the bonus amount, instead of being based on the bonus amount only. So, a 100% deposit bonus of €100 with a 35x wager requirement can end up having a €7,000 wager requirement instead of a €3,500 wager requirement.
Another common pitfall is wagering on games where the wagers do not count towards the wager requirement or only count to a certain degree (e.g. 10% of each wager counts towards the wager requirement).