I have seen a few of these and the answer always is that you aren't using the right API... but it was working like 30min ago, I logged off logged back in and now it won't load. If I need to install a new API... which ? as I tried the most recent and same error.

You have installed a version of modmenu which requires minecraft >= 1.18. In other words, you installed a 1.18 modmenu for an older version of minecraft.