It's not possible that running the FSDT Live Update could cause the simulator to not start up. The problem must be something else, which happened by coincidence.
After you restart Windows, check all the following:
- the simulator should NOT run in any "Compatibility" mode. If it is, some other addon has mistakenly set it, and this will cause problems.
- the simulator should not *usually* run as Administrator, at least this is not how it's installed by default. However, some addons might have changed this settings, which it's *usually* fine, but it might (or might not) cause problems, depending on your user account setting.
So, first try to run with NO options checked in the Compatibility tab of the Prepar3d.exe file Properties. If that doesn't work, try running it as Admin, with nothing else selected.