Since 4.26 recently came out and you mentioned you’ve been working on it for a 6+ months have you tried using the previous version of the engine that you initially built the game on to ensure it can still load?
If this is a C++ project try building/running from Visual Studio as well instead of the Epic/UE launcher. Also try doing a clean build from Visual Studio if this is a C++ project.
Also additional information will be helpful in determining the issue(s):
- What engine did you start building the game with?
- What project type is this blueprints or C++?
- Have you migrated your project to multiple versions of the engine? Ex: Started at 4.24, moved to 4.25 and now moving to 4.26?
- Are you on Windows 10?