Recently I made a hasty mistake in forcing a shutdown when it was becoming unresponsive. Previously I've been able to boot the system with no problems by starting in single user mode (to bypass the gray progress bar (which disappears after loading partially) and spinning wheel that stays indefinitely). However, now when starting up the system, it appears that pressed keys aren't recognized.
I've tried resetting the SMC (by unplugging the power source and battery and holding down the power button for 5 seconds), but that appeared not to help.
Is there another way to get to single-user mode or reset the NVRAM (such as by opening and adjusting something in the MacBook)?
EDIT: I attempted diagnostics based on comments below by trying to boot into Verbose mode and holding Option to bring up the bootloader, but nothing worked.