Foster and Garbus was able to put a block on my account accnt for A debt I know nothing about. They did it with court order, the bank told me. I made a mistake, I ignored them when they sent me a letter advising me that they were going to court. What can I do now?

Ask them what the debt is for, and take it from there.
Did you have a relationship with anyone who could have incurred the debt in your name?
You still have to pay it, sorry.
If it's a rare case of a mistake, you will still have a lot of work proving it.

Perhaps nothing, you were notified and given a chance to respond to a hearing. You would have lost a judgement by default.

You may ( or have an attorney) review the court filing and see if there is any recourse for an appeal.

If not, they get the money in the account, and if you still owe them, they may , depending on where you live, get a garnishment of your paycheck also.

Letters don't count. If you received a "summons" or similar court form signed by the clerk of court, your time to do something would have been then.

If there has been a block on your account, it is probable that a writ of garnishment has been served on the bank. You should be getting another official notice from the court telling you your options. It may be possible that you can claim some sort of exemption for these funds. If you don't get the notice by Monday, I suggest that you get a copy of the order from the bank, and then go to the courthouse and ask the clerk of court to let you review the court file.