I have the same issue. Was sieging a castle, lord came to attack, persuaded him to join, now I can't see the siege menu and I can't play any further as I'd like to. I should add, a temporary workaround I've found was wait around for a long period of time until another army comes along and if you fight them and win, the siege menu will return. This was only possible on my playthrough having stocked up on a ton of grain to not run out of food.
Edit: While being stuck, the little camp will face the direction where you click, and you can click around the map, but cannot move. Seems the player is replaced by the camp during a siege? Very annoying bug. Under normal circumstances, this is game breaking, as waiting for a very very long time for an enemy army to show up is very unlikely, and running out of food will more than likely happen before that happens. Also noticed that 2 little siege camps that both show up as my army are there instead of the one. You can see the normal siege camp in its normal location and the second siege camp sort of inside one of the siege towers.