This is unfortunately too general a problem to help much with without more details. Safe mode is a cut down version of Windows, which avoids loading a number of services, all startup programs, and all manner of other things that can cause a problem while booting. The premise is that you boot to safemode and avoid the problem, which you then use to identify and repair/remove the offending feature.
A good place to start is "did you install any new hardware or software on the computer immediately before the problem arose?"