Let's start with the obvious, which is to turn it off (fully) and turn it on again:
Remove the power lead (if it's got one plugged into it) and then also remove the battery. Wait for a minute or two, then reinsert the battery and power it up again. (I'm assuming that it's a 'normal' laptop, with just one battery, that's easily removed and reinserted, rather than one with an internal battery that's not easily removed).