I have created a NEW post because of the MANY other posts about this problem.
I have seen several posts on the Lightroom forum and this Photoshop Family forum of the adjustment brush stops working. Many of these were unsolved. A recent post on the Lightroom forum has a REPEATABLE scenario to make the adjustment brush stop working. See here in posts #9, 10 and 11 https://forums.adobe.com/thread/2082068
The scenario is as follows and as the forum post shows happens on Windows and Mac.
Use the Adjustment Brush Remove any photo - chose Delete from Disk Adjustment brush stops working (no new pins, can't select old pins, nothing in history) Select Spot Tool and create random spot/heal and immediately delete spot/heal Select Adjustment Brush and the Adjustment Brush will start working again.