Yosemite recovery mode
If you’re struggling to start your Mac, don’t panic – OS X Yosemite offers a number of different ways to boot. A full list can be found at support.apple.com, but here are some critical keyboard shortcuts to remember.
First, if your Mac falls over when trying to log in, hold down ß as you click your username to bypass login items and Finder windows, which may be the cause of the problem. Hold the left ß key when the progress indicator appears at boot to prevent automatic login.
If things are more problematic, try Safe Mode – hold ß as soon as you hear the startup tone, then release it when the progress indicator appears. You can open recovery tools by holding ç+r on startup, or press d for Apple Diagnostics or Apple Hardware Test.
If you’re forced into recovery mode, try repairing your disks using Disk Utility, or access Terminal via the Utilities menu for a spot of advanced troubleshooting. If all else fails, consider restoring your most recent Time Machine backup.