Partial crash?

I feel like my mac crashes a lot. I'm not really sure how often it actually happens, so I'm gonna start to take note when it does. Just now, spaces stopped working and I although I could still use the internet, could not move abut to other spaces. So I had to restart and loose the simulation that I was running. Also, it took like 4 tries to restart- the apple screen would come up and the little spinner below the apple would freeze, and I would have to restart again. Finally it restarted and seems to be OK. Annoying.