Today, I believe for the first time, I encountered a situation where Visual Studio 2017 (version 15.9.36) seemed to get stuck on "Preparing Solution". I don't want to call it a "hang", because the progress bar continued to animate, but it was taking a very long time to do whatever it thinks it was doing.
I had just opened the application, then selected my reporting solution, the only one I typically work with. I could see what I normally see, which was the last report I worked on, but in front of this was a small window with "Preparing Solution" and its respective progress bar. However, there were no other controls within this overlay dialog... no OK or CANCEL buttons, and no X to click to close it. From appearances, I simply had no choice but to wait it out.
After several minutes, I became impatient and decided to take action. Luckily, the first thing I tried was the last: I pressed Alt+F4 (once). The small dialog immediately disappeared and I was left to work with my solution in a normal fashion. I closed and re-opened Visual Studio, but the behavior did not recur this time.