boot up problems

For some reason, my Macbook Air will not boot up. I was using mail this morning and deleted around 500 spam emails, tried to close it down then do a re-boot. A message came up saying Mail could not move several of the emails to the Trash but i forced it to quite then re-booted. Ever since then, all I get is the Apple icon and the spinning wheel. I've left it off for a couple of hours then switched it on again and watched the spinning wheel go round for about an hour but it is not going anywhere. HELP!!