Hi Laura
If you have the user manual that came with the camera look at the troubleshooting guide for an explanation, but in short I have had this trouble also and I usualy try removing the lens and re attaching, this sometimes works.
The troubleshooting guide says it is because there is no lens attached or the lens is a non cpu one, meaning it doesnt send info on apeture and focal lenth to the camera etc.
It also suggests trying the M mode setting.
hope this helps.
Stuart
Stuart Jack, September 2009