I have just built a brand new PC and when trying to open existing PP projects software stops around 75% of the opening loading bar. I have tried importing into a clean project and it crashes PP too, I have tried rolling back to a later version of PP and still the same issue. This happens with or without plugins installed.
Welcome to the community, and sorry for the issue. Could you let us know in which version of Premiere Pro was the projects created? Do you see any error codes or messages while importing your old project to a new one? Try launching Premiere Pro > Window > Workspaces and uncheck Import Workspace from Projects, and see if that makes any difference.