problem starting up Imac
Problem starting up Imac
Problem starting up Imac
Without any more information than that all I can suggest is to read this About the screens you see when your Mac starts up - Apple Support
And maybe this Resolve startup issues and perform disk maintenance with Disk Utility and fsck - Apple Support
Try resetting the SMC Resetting the System Management Controller (SMC) on your Mac - Apple Support
problem starting up Imac