Be sure that if you moved a lot of files to an external hard drive, then put THOSE files in the trash from your internal drive. They STILL take up space, even in the trash, so be sure to empty the trash, as you don't really accomplish anything until you actually empty the trash. If the hard drive is completely filled, even emptying the trash may not be do-able.
Absolutely restart into Safe Boot mode, by restarting while holding the shift key. You will boot to a login window, even if your Mac normally bypasses that login window. Log in. Safe Boot mode MIGHT give you enough "wiggle" room to empty the trash, and your first task is to empty the trash - then immediately restart. If this allows you to go back to normal work - do NOT continue until you verify that there's enough space for the system, plus your normal video file use. Looking to the future, a good rule of thumb is a minimum of 15% of total space left free. If your Hard Drive is 320 GB, then try for at least 48 GB free.
Serendipity is a lucky guess !