swapping hard disk problems

I had Linuxcnc working fine on my old old PC but then the machine failed. I acquired another ancient machine and thought I would just swap the old hard disk into the new machine. This kind of works, in that it boots up and gets me to the page where I have to choose Debian. However, whereas I would then previously have had strings of text scrolling across the screen as linux loaded, followed by a log in page, I don't see anything. The HD is whirring but nothing is appearing on the monitor. I'm pretty sure I am getting to the login page because if I type in my username and password, the HD goes into action again, and I get to what I presume is my desktop, except that all that shows on the screen is a single brown rectangle. I realise this isn't really an installation problem but I feel it ought to be quite easily fixable if I knew how. 

Thanks actually I solved it by putting "Nomodeset' into the grub bootloader ... I have no idea what this means but it seemed to work ... although I then had to do something with the power saving settings