Whether you remember touching the brightness control or not, check that the front-accessible brightness control is not fully anticlockwise. A quote from Apple for AASPs:
'Screen is dark; audio and drive operate:
1 Turn brightness control clockwise.
2 Check video cable connections.
3 Replace analog board.
4 Replace video board.
5 Replace logic board. Retain customer’s SIMMs.
6 Replace CRT.'
de
'Screen is dark; audio and drive operate:
1 Turn brightness control clockwise.
2 Check video cable connections.
3 Replace analog board.
4 Replace video board.
5 Replace logic board. Retain customer’s SIMMs.
6 Replace CRT.'
de

