It depends on the bonus. In most cases, if you have an active bonus with pending wagering requirements, you will not be able to withdraw the bonus funds until those requirements are met.
Your real-money balance (funds not tied to a bonus) may still be withdrawable. You can check which funds are available for withdrawal in the Withdrawal section of your account.
To track your wagering progress, go to My Account → Promotions → Active Promotions. For full details, refer to the Bonus Terms.