What a pain! While Premiere sits on the splash screen loading all of the plugins and things, is there a plugin in particular that it hangs up/ freezes on?
As we try to figure this out, sometimes when silimar things happen to me, I try opening Premiere Pro up from the app itself. Then I go to File > Open and select the project. For some reason occassionally that works.
When all else fails I hold the Alt key down to reset preferences. Usually it opens Premiere a little better. Resetting the preferences is annoying to do since things will be reset, but sometimes necessary to get a project open.