ToME: the Tales of Maj'Eyal

I turned level 30, selected stats, generic and class points and then exited the level up screen. I forgot to pick a prodigy, and now I can't go back in and select one. I had to wait a level to do this. The level up button should stay on until a prodigy is selected.

You should be able to open the level up screen at any time.
Even if the button in the interface is disabled, check the keybindings and use that to get in.