Hmm, these are the possible causes of your problem - take your pick. I would not know from here where to start:

• Missing or corrupt system files
• Missing, corrupt, or incompatible drivers
• Corrupt boot configuration settings
• Corrupt registry settings
• Corrupt master boot record, partition table, or boot sector
• Improper installation of Windows

If you had done imaging, it would be easy to get you back on track. But without it, you may be facing a reinstall.