Starting up in Safe mode runs various process designed to return your Mac to good health. It checks your hard drive, loads only the system files, removes font caches, and disables startup and login items. If your Mac sucessfully starts in Safe Mode the chances are your issue is software related.
Run Disk Utility
While in Safe Mode navigate to Applications/Utilities/Disk Utility, double click to open. Choose the First Aid tab and then highlight the hard drive partition (Macintosh HD unless you have changed the name). Click on Repair Disk Permissions and wait for the confirmation that the precess has finished in the white log window in the centre.
With the above 2 steps complete, restart your Mac. In many cases this should be enough to have your Mac running happily again. If not then please do give us a call.