2
Could it be, that you accidentally turned on the mask?
You could try pressing Alt + M to clear it and see if you than can sculpt again.
2
You need to recalculate the normals: in Edit mode, select all and press ShiftN
1
You want to check Accumulate under the Brush Menu in the top of the 3D view interface while in sculpt mode.
Or in the Tool settings in the properties editor :
It's the same setting, just accessed from two different places in the interface.
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