Holding the power button down to see if you can get to a (usable) 'Power Off' option. If so, simply use that and then (after waiting for a short while) hold the power key down again to turn the device back on.
If that doesn't work, follow the instructions in the first part of my 'hard reset' link but, instead of choosing 'Wipe Data / Factory Reset', choose 'Reboot System' now.
If that doesn't work, prize the back cover off the device, disconnect the battery, wait a few moments and then reconnect it. (i.e. follow Steps 1 and 2 in this link but use the first part of Step 3 before reconnecting the battery again:
https://www.ifixit.com/Guide/Tesco+hudl2+Battery+Replacement/102697 )