Use redsn0w: Usually the easiest way to reinstall Cydia is to download and open the latest version of redsn0w, click "Jailbreak", make sure that "Install Cydia" is the only checkbox checked, and let that process complete.

Use iFile: If that doesn't work (such as if redsn0w gives you error messages), another easy option is to use iFile, if you already have it installed on your device. In Mobile Safari, go to this URL: http://apt.saurik.com/debs/cydia_1.1.8_iphoneos-arm.deb - and use iFile's option to install it.

Use the AutoInstall folder: If you don't have iFile on your device, you can use a desktop application such asiExplorer, DiskAid, or Phone Disk to access the device's filesystem and install Cydia. First open the application and navigate to (or create, if necessary) this directory: /var/root/Media/Cydia/AutoInstall - and then download the following file and put it in that directory: http://apt.saurik.com/debs/cydia_1.1.8_iphoneos-arm.deb. Reboot your device, and Cydia should be installed.

Use the command line: Or if you prefer a more elegant method, you may be able to SSH into your device and reinstall Cydia. You need to already have OpenSSH installed on your device, along with APT 0.6 or 0.7 (which are dependencies of various packages including CyDelete). After SSHing into your device: type apt-get install cydia and then type su mobile -c uicache (or reboot your device) and you should have Cydia again.