When laptop turned on, and Ubuntu is chosen for OS, I get a blank purple screen and can't log on. How do I fix this problem?

Question

I turn on the computer.

I can select either Windows 7 or Ubuntu.

When I select Windows 7 it works well.

When I select Ubuntu, it normally works well.

But right now when I select Ubuntu,

It leads to a blank purple screen, with no mouse, and no log-on option.

Recently my computer turned off in the middle of the session,

Charger got disconnected.

But that's happened before, and it didn't cause the computer problems turning back on in the past.

I'm wondering how I can access Ubuntu, and if you can help out.

Thank you.