I've been working pretty hard on this first sculpt, using all the Sculpt tools with a multires modifier on the mesh. All of a sudden, it seemed that the Crease Tool (which had been working before) has no effect on the mesh! All the other tools seem to still be working. What am I doing wrong here?
This is not a bug rather than expected behaviour caused by the empty texture applied as a sculpted mask:
The mentioned option is in the brush settings on the Toolshelf > Texture rollout.
Note that there isn't anything assigned to that texture so it's counted as black which means zero effect of the brush (while white is full). Probably that texture was created unintentionally when pressing on the New button accessible if texture isn't assigned to the brush.
To retrieve usual brush effect delete texture from the brush settings by clicking with mouse on the 'X' button next to the texture name (or assign an image file to the brush in the Properties editor > Texture tab).