Not exactly sure what you did, but if you can't read the screen, there are only a few options.
1) Press and hold the power button until the tablet shuts down. Then restart it to see if the tablet returns to normal
2) Attempt a Safe Mode restart
3) Connect the tablet to a PC and see if you can backup your data
4) Factory reset the tablet. You will lose all data when you do this, so do #3 above first
Unfortunately, like many people posting here, you haven't posted what device you have, what OS you're running, or what carrier you're on. Search Google for how to start your device in Safe Mode and how to get into Recovery mode if you need to factory reset the device.