Whats wrong with my computer?

My Desktop computer was bought around 2001 its very old till this day but it works marvelous like it is still brand new my only concern is when i turn my computer on, it goes into sleep mode like when you don't touch your computer for a good 20 minutes and it enters sleep mode, thing is when it enters this phase it doesn't want to operate so i would have to turn it off manually and back on a couple of times before it starts to load the windows logo. please ask me for any extra details and ill try to help you with that but please if you knew what wrong with my computer please tell me and how i can fix it.