Long Story Short:
Issue: editing cmdline.txt was the issue of RPi3 not booting
Solution: plugged SD card into an adapter (Polaroid Cube+ in my case) that could read the card and used Ubuntu to revert the cmdline.txt card
Original cmdline.txt contents: dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 root=/dev/mmcblk0p7 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait
I was following this tutorial to use a GPS with the RPi3. After I changed the cmdline.txt file and rebooted, RPi3 didn't reboot. I thought I shut the RPi3 down instead so I unplugged and replugged RPi3. It still didn't boot up after so I thought I messed up the SD card.
Initially, when I inserted the RPi3 micro SD card into my laptop, the card reader did not recognize the SD card. I then started to use different card readers and ended up inserting the RPi3 SD card into my Polaroid Cube+ and then connected my Polaroid Cube+ to my laptop. My laptop recognized the SD card that way; however, it only saw the "RECOVERY" partition. I didn't see "cmdline.txt" in there so I googled for different solutions.
Second attempt was to put the SD card back into the RPi3, press SHIFT on the attached USB keyboard while RPi3 boots to go into Recovery mode (NOOBS). Once I got into Recovery mode, I clicked the "Edit Config" icon. That allowed me to change some configurations on "Boot" and the "cmdline.txt". I copied the default cmdline.txt configurations from here: http://elinux.org/RPi_cmdline.txt and rebooted. This did not fix the issue; however, it got me one step further. In addition to the "VFS Unable to mount root fs on unknown block()", it gave me something like "entering kdb on processor 2 due to keyboard entry."
Third attempt worked like a charm. I was panicking because I didn't have any backup laptops or computers, but then I remembered that I have Ubuntu installed on my VirtualBox. I started Ubuntu and then attached the SD card (using the Polaroid Cube+) to Ubuntu virtual machine. HOORAY!!! Ubuntu read ALL the partitions on the RPi3 SD card!!!! The four partitions are root, boot, RECOVER, and SETTINGS. First thing I did was copy my code for backup. Second thing I did was look for the cmdline.txt file. Luckily, I made a backup of the cmdline.txt file in the same directory before I changed it. I replaced the cmdline.txt file with the backup file, inserted the SD card into my RPi3, and the RPi3 BOOTED AGAIN!