While Macs have a great reputation for being stable and rarely experiencing major issues, the reality is that sometimes things can go wrong. Typically this happens when either a hard drive fails or an OS X system update goes completely awry, but if you have set up Time Machine backups on the Mac like all users should, then you will discover that restoring an entire systems hard drive from that Time Machine backup is really quite easy.

Source: How to Restore a Mac from a Time Machine Backup (OS X Daily)