First try restarting (turning off and back on) the device. That may well resolve the issue.
Should that not help, try going into "Settings" and then "Language & input". The option to choose what keyboard you are using is in there. As long as there is one listed then the device still has one.
If it still is giving you issues, then you may want to try starting it up in "Safe Mode" and see if the keyboard works fine in there. If it does, then there may be something on the device you will need to remove. Bad app, virus, malware, etc.
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...