Tekkit crashed after "Downloading terrain".

[MrBean007]

[Curunir]

Dimensional Doors is notoriously unstable. Your chances to get back where you have been are nil, I think.

As intriguing as it might be, refrain from building in alternate dimensions. The Overworld, Nether, Moon and Mars are stable enough, but neither Dimensions nor Mystcraft Ages are. If you still want to give it a try, make very frequent backups of your whole world save, while it still works, and be prepared to fall back onto those saves a lot.