Uninstalling and reinstalling worked for me.  I restarted after the uninstallation and before the reinstallation.
I am finding that SupportAssist no longer works after the April 10 Windows Update. I have a Dell 8920. It seems like it also has to do with some stupid new plug-in where Dell couldn't help but try to upsell you to antivirus protection. It was just hanging, so I unistalled and reinstalled, and now it doesn't even appear when I run it. I really, really, liked that SupportAssist app before that. I just deleted every Dell software on my computer because it was making things unstable. I guess I am just at the mercy of Windows Update now and not the device-specific expertise of Dell. I am extremely disappointed. Is anyone else having this problem?
Support Assist no longer installs or works after April 10 Windows Update