You need to go into recovery by holding power button for a while during startup.
Then I would try to reinstall MacOS if you haven't recently backed up your Mac, if you have recently then I would suggest a wipe and reinstall.
You are having a boot up error most likely due to corrupted software. It could be hardware related as well but if you wipe and reinstall it should solve the issues and if it doesn't then you have to take to an Apple store.