Don't be afraid of the command line, breaking Linux is how you end up learning how to use it!
I haven't done this tutorial but if that kind of thing helps you this one looks pretty good.
My best guess is you need to do something like:
(In the shell, one line at a time, enter runs the command)
mkdir /mnt/tmp
mount /dev/sda2 /mnt/tmp
nano /mnt/tmp/etc/fstab
Nano is a text editor that uses your whole terminal, so you will see the contents of /mnt/tmp/etc/fstab (the file that controls where disks are mounted) and replace 'sdb' with 'sda' on the line starting with /dev/sdb2. The bottom of nano's screen shows you the keyboard shortcuts, I think Ctrl W will make it write the file, asking for confirmation of the filename, which should stay the same. Exit nano (Ctrl+x maybe?) then reboot with the command 'reboot'
If you get any errors about access denied or permissions, run 'sudo bash' to get a shell with more power and try again.
Good luck!
What most likely happened is your disk order switched and, as others have mentioned, using /dev/sda1 or something similar to point to partitions is unstable and can't be trusted. Once your system is back up, look up how to specify partitions in /etc/fstab using UUID (something like /dev/disks/by-uuid/xxxx-xxxxxxxxxx-xxxx instead of /dev/sda2)
