The Automatic Stay
When you file for Chapter 7, an automatic stay goes into effect. Some exceptions exist – creditors have the right to petition the court to ask that the stay be lifted – but it typically begins as soon as you file and it remains in place until you receive your bankruptcy discharge. After you file, your creditors are prohibited from taking any action to collect from you, including freezing your checking account. This is true whether it's solely in your name, or if it's a joint asset that you hold with someone else.
Even if your creditors can't reach your joint account, your trustee may be able to. Technically, he can take half the money – your half – to pay your creditors. However, you get exemptions when you file for Chapter 7, which is property that you're allowed to keep. If you use an exemption to protect your half of the checking account, the trustee can't take it.