I've got a 2011, 21.5" iMac with a 1TB hard drive (likely Seagate, although I don't think I can check right now). After it froze, I did a cold restart by holding down the power button for a few seconds. On restart, the machine produced the startup chime, and then froze, with the loading wheel spinning under the Apple logo. This cycle seems to last indefinitely.
I tried resetting the SMC by unplugging the computer for 15 seconds, then restarting it 5 seconds after putting the plug back in, but that didn't seem to help. I then booted into Recovery HD (the volume selection screen accurately shows all volumes, including the internal hard drive), and verified Macintosh HD, which, as it turns out, is OK.
After another failed attempt at a restart, I tried booting into single user mode and using
chmod -R -N /Volumes to try to reset the ACLs, but it only tells me that all my volumes are read-only.
Can I do anything short of returning it to an Apple Store for maintenance?
EDIT (this might be important)
At no point in the boot does the internal hard drive make any sound. No head clicking, no spin-up, nothing.