Setting brightness to minimum results in black screen

[Joseph Wakeling]

I was playing with brightness settings on my new BQ Aquaris phone, and slid the screen brightness down to the absolute minimum in order to try and reduce battery usage. This didn't work, because the automatic brightness adjustment was still selected; but when I un-selected that option, the entire screen went black and has not recovered since.

Pressing on the power button, as if to wake up the phone, occasionally results in a brief flicker of what looks like the touch login screen, before the screen goes black again.

This obviously seems to me as a bug (minimum screen brightness shouldn't be completely black!) but in addition to a fix, I'd really appreciate someone advising how I can reset the phone's settings to defaults, given that as things are, I cannot actually log into the phone and adjust things. (Ideally, reset phone settings without killing any of the data already copied to the phone.)