1. Check the power adapter and the power outlet. Make sure the power adapter is plugged in securely and that the power outlet is working.
2. Reset the SMC (System Management Controller). To do this, shut down your Mac, then press and hold the power button for 10 seconds.
3. Reset the NVRAM (Non-Volatile Random Access Memory). To do this, shut down your Mac, then press and hold the Command + Option + P + R keys for about 20 seconds.
4. Try to start up your Mac in Safe Mode. To do this, shut down your Mac, then press and hold the Shift key while you turn it on.
5. If the above steps don't work, you may need to take your Mac to an Apple store or an authorized repair shop for more in-depth diagnostics.