It may depend on where you rest your hands on the keyboard.
On my laptop there is a key (actually 2 keys) for adjusting brightness: Hold down the "Fn" key (in my case this is bottom right and Fn is coloured blue) and at same time press another blue coloured key looking like a symbol for the sun, but empty in the middle. (I know this sounds weird, you'll recognise it when you see it). To darken use the same process but press the "solid" sun symbol.
Maybe you happen to have pressed these keys withourt realising it.