First: Try a new game. A real new game, pressing the new button.
If the MCM is now working, you have proven the save used, has an incompatible load order, and may also be corrupt.
Example: If Toys was added to your load order for the first time, starting a new game is required (see this FAQ for why), otherwise you are producing a guaranteed corrupt save, plus there's a good chance MCM will not show. When MCM does not show, Toys has shut down, and many things related to Toys are not going to work.
If starting a new game does not make your MCM appear, read on.
Max mods
SkyUI can load a max of 125 mods (mods that have MCMs). If you exceed that, some mods will be missing their MCM. If that's your case... working as intended. We don't recommend mods that try to work around this. Use those at your own risk.
"MCM menu not showing up" is an ages-old SkyUI bug which is vulnerable to load. The following are the tricks you can use to potentially work around it.
A. If you are not already, use an Alternate Start mod.
Vanilla game start is well known for new game MCM issues. Alt start mods are in part designed to allow for big mod load orders.
B. Start New Game Twice Trick
Start a new game
Wait 30 secs
Exit to the main menu
Start a new game again
Works in some situations because Skyrim experiences less load, loading in part, from cached memory.
C. PostLoadUpdateTime:
Open Skyrim.ini (My Documents/My Games/Skyrim)
note: location of Skyrim.ini can vary
If using MO2, your Skyrim.ini location might be Users%user%\AppData\Local\ModOrganizer\Skyrim Special Edition\profiles\Default
Look for the [Papyrus] section and edit as follows, or add the section and/or line if not already there
[Papyrus]
fPostLoadUpdateTimeMS=2000.0
This will allow for faster load times. Good if you have a large number of mods.
D. Kick Start the MCM
in console, type setstage SKI_ConfigManagerInstance 1
This does the same thing as "Jaxonz MCM Kicker " mod, which is unnecessary if you do this.
E. Change up the Load Order
Try moving Toys.esm as early as possible in your Load Order. Using the "early loaders" group in Vortex is a good method if using Vortex, or you can manually do it. If Toys.esm is already early, try the opposite, making it one of the last ESMs to load. The point of this is to cause different timing finding a better time to load so SkyUI does not break.
F. Temporarily Stop OverClocking FPS
MCM startup is unreliable using a higher frame rate than the default (60 FPS). Simply limit your FPS to 50 during new game startup, then switch it back after.