Did you make sure to turn off the phone before trying to get into "Safe Mode"? If not, and there is something fishy on the device, then it won't work. Below are the instructions for doing so, just in case.
Now if it still won't work, then you would next either try a "Hard Reset". Which would sadly wipe all personal stuff off the device, or have it looked at by a tech, who still may have to wipe the device.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you...