You need to perform a factory data reset. I did this by going into recovery mode (Power button and volume down, release the power button after the logo appears, release the volume button after about 5 seconds). Choose wipe data/factory reset and then choose wipe cache (maybe optional) also. Please remember this will delete all data from the tablet, so if you don't have a backup, you will lose your data.
Choose reboot after this and it should boot up normally (takes longer than usual). This worked for me