I recently updated the Cydia package installer, and something went wrong. It crashed back to Springboard while updating itself, and refused to start up again.
All is not lost however. Remember that Cydia is actually a graphical front-end to the iphone port of apt-get, so you can still install applications via the command line. So drop down into Terminal and issue the following command :
apt-get install cydia
If you receive a permission denied, you are working as user ‘mobile’ and you need to be user ‘root’ to do this. So before you type in the above command, type ‘su root’ and the root password and then the apt-get command.
Of course, if the problem is the install package itself, this will not solve it. I tried the above a few hours later, and there was already a new release waiting to be downloaded, and this fixed the problem.