The Live Update cannot possibly caused that.
If the antivirus blocked something, the worse it could happen is the Addon Manager/Couatl might not start, and so GSX and all the FSDT airports wouldn't run, but it's shouldn't cause the simulator to hang. Well, unless it tried something really dangerous: like trying to "heal" some files from a non-existant threat and, if the affected file was an executable, than changing even a single byte in it, will likely crash the sim.
When the sim crash hard, sometimes Windows automatically sets a special compatibility flag in the registry, but this on FSX or P3D has the opposite effect, and prevents the sim from starting, even after a Windows restart, because the setting is stored in the registry.
But, we CLEAR that wrong set in registry with the Live Update so, it might have been enough to just run Live Update again (once you are sure the Addon Manager folder is excluded by the antivirus), to have the sim restarting again normally.