Ubuntu on Lenovo x61s

[Peter]

I  have ubuntu installed on an Lenovo x61s laptop. Yesterday the 
systembooted and worked without any proble. Today it boots as far as 
asking for the encryption password. It then comesup with
  BusyBox v1.21.1 {Ubuntu 1:1.21.0-1ubuntu1)built - in shelol (as)
enter help for list of built in commands
(initramfs)


What does this mean and how can I get the system to complete booting?

Regards

Peter Goggin