If Cydia is crashing upon load, you can try these steps to fix it. These steps require iFile installed, ssh installed, mobile terminal installed, or an understanding of how to get at the file system with iphonebrowser or such. Those topics are not covered here (look for another guide for how to get to the file system elsewhere such as the ssh guide here).

This guide should only be used if cydia crashes upon load. It is not to solve any other problem.

Locate and delete the following two files:
/etc/apt/sources.list.d/cydia.list
/var/lib/cydia/metadata.plist

Remove anything in this folder. Leave the folder, just make sure it is empty:
/var/lib/dpkg/updates/