HELP ME!?Computer won't boot. Maybe because memory is full?

When I start up my laptop it says " We apologize for the inconvience, but Windows did not start successfully. A recent hardware or software change might have caused this." I tried Safe Mode but it still wouldnt boot. I did get a warning last time I was on that said I was running low on available space. That's the only reason i can think of that would make it do this suddenly. If you know how to fix this problem (and could explain it me like im 5) that would be great. Thanks