Try restarting the machine and pressing F8 at boot up.
From that list, choose 'Last known working configuration.'
Does that help?
If not, when the machine is trying to start, press and hold down the power button until the machine shuts down.
Restart the machine and a screen should come up giving you a choice to start normally or repair your computer.
Choose repair.
Try to cancel the repair process, you have to be quick, or you will have to wait for some considerable time.
Cancel and shut down the repair process and you should arrive at a screen where you can choose to 'Restore' your computer.
Try to restore to a configuration that is some weeks earlier and let us know how you get on.