Once the tax return has been filed and accepted by the IRS the bank routing and account numbers cannot be changed.
If the bank information is not correct the bank will reject the direct deposit of the refund. The refund is sent back to the IRS and they will then mail a refund check to the address entered on the tax return.
If the bank account information on your tax return is not a valid account, the IRS will mail you a check, as DoninGA said. If it's a valid account, but not your account, your refund might be deposited in someone else's bank account. (Some banks check the name on the deposit against the name on the account, but not all banks check.) If your refund has not been issued yet, you might be able to stop the direct deposit and have a check mailed. See the following page on the IRS web site for more information.