It would help if we knew what happened before this (e.g. a drop, a software update, or maybe nothing special)? Also what phone you have.
Without that, all I can suggest is that maybe clearing cache or a factory reset will help. To do this you need to get into recovery mode. This is generally done by booting while pressing some combination of buttons, but which buttons depends on the phone manufacturer (sometimes model), so without that information I can't tell you exactly how. I'd try wiping cache first, then reboot and see whether it's any better. If not you could try again and do a factory reset (which will however erase all user-installed apps, data and settings).
If that doesn't work (e.g. if the problem is that a failed software update has corrupted the ROM) then you may need to reflash the phone's software completely. How you do this depends on which phone model, so if it comes to that please post again and tell us what device you have.
Good luck!