Sounds like your Boot Manager may be corrupted. Download and run Recovery Media and choose 'Startup Repair' this should repair whatever is damaged in the Boot Manager and get you back up and running.
If that doesn't work you might want to simply re-format the machine and load on a snapshot tool like Rollback Rx or Comodo Time Machine. It could be that if you've updated or installed something recently it's causing this, so testing with a snapshot tool makes sense.