When I tried booting into recovery mode (Cmd ⌘ + R) during boot up, there's a field asking for a password. I do not know this password.
I am using a late-2009 21.5" iMac.
Anyone know what to do?
From the image, it looks like a Firmware password.
This bash script may be able to decrypt your firmware password, however it only works on a limited set of computers.
python -c "print ''.join(chr(int(c, 16) ^ 170) for c in '`sudo nvram security-password`'.split('%')[1:])"
If your mac is pre 2011 then removing a strip of ram and rebooting will clear the problem. I have done this with quite a few that I have installed new systems onto and it seems to work very well. Give it a try, what have you got to lose.