iMac won't start up after reinstalling os x

On Friday night I told my iMac running maverick that it could restart to install updates. Later I found it showing error messages and wouldn't start. I spoke to support and we ended up having to reinstall os x . She told me it should be fine after. Now that finished and it e started, only to stay stuck on the boot up page. It's grey with the apple and the loading circle going around. Support is closed now and I don't know what to do next. I'm supposed to be working on this computer tomorrow!

Would appreciate any help,

Thanks!