Need more details.
What version of macOS are you running?
Have you made any hardware or software changes recently?
Did you experience any odd symptoms prior to today's boot?
What GPU do you have installed?
Is your recovery partition bootable? Boot the Mac, hold down Command-R as soon as you hear the chime and keep both keys held down until you see it booting. From there you can try running Disk Utility and try the First Aid process.
If your drive checks out as OK you may need to re-install macOS on your drive. Doing so should preserve all of your files but if your data is super important and not backed up anywhere then your first order of business may be getting the important data backed up in case the drive is in the process of failing or something goes really wrong with the reinstall.
As an aside:
If your valuable stuff is not backed up, then I would argue it's not that valuable to you. You can probably recover it in this case but you should never put yourself in that position. External USB hard drives (even large ones) are cheap these days. There's no excuse not to be doing regular Time Machine backups on your Mac.