My sister and I played on a local multiplayer server then suddenly her game crashed (she was hosting). When she opened our world almost half of what we made was gone. We did nothing else but build a base (she thought it was because I destroyed her sign while she was writing on it). Is there any way for us to bring back our world?
No, You may not bring back the previous save of your world, as it wasn't backed up, and while you crashed, everything returned to the newest save.
The automatic save feature only saves between a certain time, It can't save EVERYTHING you do every second, It will just lag the server, and lag your CPUs.
So, no, You can't restore it, but as a future advice, You can try to save more often, In order to save the world, You must be an OP, and write /save Either in the console, or in the chat.