I have a late 2010 27" iMac running the latest version of Sierra. There is approximately 70% of the hard drive available and it has 8gb ram. Yesterday my computer was running particularly badly with the spinning ball throughout the day which would last for several seconds. This was happening in all apps. I rebooted twice but it did not improve. I decided to try Disk Utility. On scanning my main drive I was told it found errors and required me to try First Aid in recovery mode. I did this and it could not fix the error telling me 'First Aid process has failed, if possible back up data on this volume'. After a couple of failed attempts I restarted the iMac but it will no longer boot. It gets halfway along the progress bar on startup after chiming then just switches off. I tried unplugging all peripherals but that didn't help. I have followed instructions to reset the NVRAM but that hasn't helped either. Luckily I do keep backups. Any suggestions what to do next as currently it is pretty much a brick!