stay calm. Never easy to figure this stuff out remotely but if we approach this systematically we can probably figure this out. first:
Please tell us your system specs: OS version, Premiere version, amount of RAM, Hardware specs including graphics card and your source properties and sequence settings including codecs, pixel dimensions and frame rates.
Often as projects get more complex things can get problematic particularly if your system doesn't meet minimum specs.
Also, projects can get corrupted. One workaround can be to create a new project and rather than opening your problematic project, import it into the new project.
It's also possible that one of your hard drives is getting too full. You should leave at least 10% free space on all drives and at least 20% on your startup drive. And it's possible that one of your hard drives is failing. Are you running any other programs at the same time? Don't. Have you tried restarting your computer? Resetting Premiere preferences and caches? Have you installed any new hardware or software recently?