You don't say what model phone you have, but that sounds like you are in recovery mode. Try pressing power and volume up together, see whether that brings up a menu (it would on an HTC when showing this).
If the phone is booting straight into recovery that's not a good sign - it suggests that something has been corrupted. If you can get a recovery menu you could try wiping cache, then reboot and see whether it's any better. Or if that doesn't work, you could try a factory reset (which will erase all of your apps and data). If the problem is a user app or data/settings, that should fix it. If it's that the system software has somehow become corrupted, it won't - in that case you'll need to load a fresh set of system software.
But before any of that, just try pulling the battery out for a couple of minutes, to give everything time to fully discharge, then see whether it is any better. Probably won't be, but sometimes you get lucky (the problem is a stuck bit somewhere, and this clears it), and it's easy and doesn't risk losing any data.
I can't tell you how to reload the phone software because I don't know what phone you have (network branding can also make a difference). If you want more detailed advice on that, the best would be to sign up so that you could ask in the forum here for your device - that way you can discuss it with someone who knows that phone model.