There seems to be thousands of post related to my problem but every solution I have found requires access to the GRUB which I cannot seem to do.
My problem is that after I installed Linux Mint 17 from USB which booted and worked fine (even with three monitors) I just get a black screen with a blinking cursor in the top left of the screen - a problem which I have read about and to fix it I need to do something like the following: viewtopic.php?f=46&t=122257
The problem is I don't have access to the GRUB, it just boots directly into the Linux partition and then I get the black screen.
Any help fixing this would be greatly appreciated! I've literally just done the Linux Mint 17 install on my laptop (that im using now) and it all worked a treat. My computer has a Radeon HD 5770 Graphics card which I assume is the root of the issue.
Thank you all for helping out, greatly appreciated!