Try an SMC reset and NVRAM reset.
How to reset the System Management Controller (SMC) on your Mac - Apple Support
How to reset NVRAM on your Mac - Apple Support
You may try starting up in Safe mode by holding down the Shift key while starting to see if your computer will boot.
Also, you can try Recovery -- hold down Command and R keys on booting -- and use its Disk Utility to check your startup disk.
If you have an Apple store or approved retailer nearby it may be best to take it in for an evaluation.