There are several reasons it could lead to a sudden problem with FNV in MO2
1. You might have misclick on the right window panel and sorting it based on alphabet instead of priority number. This could lead to unloadable some of the case, but in other case it could load alright, but buggy in other state. Solution? rearrange the right window panel based on priority number, not alphabet.
2. You might have do some tweaking in Geck. After saving, most of the time the just-saved esp or esm will be kicked down to highest priority, lowest spot in right window panel. If that esp or esm do too many strange things, it could have broken your sessions and unloadable. Rearrange said esp/esm back to its proper spot (if you remember) to see if it start again. If not, you might want to disable it to see if it helps. if starting in that case, then you need to check said esp/esm to see what you did to create that kind of problem.
3. You might have installed some new mods that extremely incompatible with your rig, from software to hardware. Or you could have upgrade your driver and it's extremely incompatible with your mods and your rig. Case in point: AMD latest driver upgrade has some hickup with nvse. Another case in point: last year, or year before last, nvse did a full rewrite in code, which lead to some breakage in some pluggins, which lead to some mod creating unloadable and or notworking any more. Say, sup pluggin, and its two dependant mod that utilize screenshot function. In which case, you might want to test this shit one by one to see which create trouble.