Did you have a previous version of MC on this PC? Which version?
The most likely scenario is that the previous version, if there is one, is still set to start with Windows. If you did the upgrade to MC24 but didn't reboot the PC more than once, MC24 could run well for 3 or 4 days without the older version being started. If, for example, the PC was just sleeping rather than being shut down at night.
Two possible fixes, if there is a previous version still on the PC.
1. Make a backup of your MC24 Library, then close MC24. Uninstall the previous version. Reboot. If necessary (though it shouldn't be) run the MC24 installation program again to fix any issues.
2. If you don't want to uninstall the previous version, maybe because you haven't checked everything yet, then run the old version and set the options at "Options > Startup > Windows Startup > Run on Windows startup" to "Nothing". Then run MC24 again and check the same setting. Then reboot and test if MC24 works.