I had an old macbook pro that had been sitting around for maybe 6 or more months, and I tried to turn it on to use it, and the first thing I noticed was that 1) it was mostly frozen, and 2) the trackpad was not clicking. This made it hard to use, but with a little keyboard navigation I was able to google.
I googled around and found some suggestions—mainly that it was probably from a bad battery (which was probably the case), but also a fix which worked for me.
The fix was to push down hard on the trackpad evenly and hold down for about 10 seconds.
After this it pretty much started working again.
Just wanted to add my data point back to the internet.