Clean install - but won't boot

Clean install - but won't boot

I have installed Ubuntu via USB with the clean install option - to wipe
everything and install to the HDD.
The install goes just as the tutorial says it should...I click restart
when it has finished and:
It shows POST Then it's just a black screen with a flashing cursor in the
top left.
I've tried to install it again - I've tried with both v12 and the new v13
and it does the same thing.
What's happening and how can I fix it?
Thanks very much in advance.