stops and starts, gaps appearing

I am having a lot of trouble with my video making.  Everything was working fine, was rendering, etc., but now, all of a sudden,

it's throwing clips off, making gaps in the video, but keeping the audio, won't let me close gaps, stopping and starting, just a mess,

and this project is due in one week.  I've been working on it for 4 weeks.

Where can I get some help?