When iMac is not starting up, it is better that you perform a power cycle. Leaving the system plugged in for a long time also affects the battery and hence you'll need to troubleshoot the power adapters. Normally, Mac not starting is the problem users face when they use incorrect power cable or adapter for the MacBook. Test the outlet with other electrical lamp or phone charger to see if it is working.
The most plausible reason would be that battery drained from your system and as a result, Mac laptop won't turn on. Just follow them in the same order and one of them will work out for you. Take a look at the solution and try them to see which one fixes Mac without any hassle. Even if you can't figure out the exact reasons, trying out the common solutions won't harm Mac.
It is basically the hardware issues that lead to MacBook pro not starting issue. Broken hard disk permissions are conflicting with user accounts.The corrupted cache of critical system files.Incompatible login items related to application interrupting the Mac boot process.Broken application affecting the system startup.It's a display issue instead of a power problem.There isn't enough battery left on the Mac.If MacBook won't start or boot, the main reasons might be: