OP, do you have medallion DL status? If so, the bulkhead seats become available at exactly T-24, when you are able to check in for the flight online. You don't have to check in at that time to be able to change your seat to the bulkhead, however. If you do not have status, you should still watch for those who do to move to bulkhead seats right at T-24 (to the second).

Also, whether or not you are a medallion, you should frequently check J availability to see if seats are still available. If so, pax may opt to use miles/SWUs to upgrade from Y to J, or might be offered the option to pay to upgrade the flight. As this happens, their former Y seats become available. Checking the seat map very frequently starting at T-24 will often increase your chances of snagging a seat when one opens up.

You still should have been given IDB compensation whether or not you are a DL medallion. If you are confirmed on the flight, they are required to get you on the flight, even if you do not have an assigned seat. What were you given as compensation for being denied boarding on your confirmed flight?