Screen freeze.

[nannakath]

I am stuck with an email unfinished and can`t move-my finger slipped and I`m getting a square appearing at top left and because of the `egg-timer` symbol it won`t allow me to cancel the `square` so I can`t complete the email and can`t close computer down. Anyone available to help me with this please?
Many thanks.

[nannakath]

I didn`t receive any answers to this and it happened again-stuck and can`t move so I have to switch off and on again and retrieve the email I began from `draft` but I still wonder if I can unfreeze without switching off computer.
Any experts around with help appreciated. Thanks.