startup and shutdown problem

My Windows 98 worked fine until recently.It couln not shut off unless I pulled the plug.Also,on startup the monitor will not go on. Then I pulled the plug and wait a few minutes.It started up Ok.It does that once a while.What is my problem?
Win 98,20 Gb HD,256 Ram,CDR-W,DVD,floppy,zip drive.,Pentium III.

Jerry Gilles