have you hidden it with poof? or with SBSettings?
an easy way to get it back is restore+re-jailbreak!
i have never had cydia just disappear on me, so you must have a program hiding cydia, or something has just gone wrong!
1st go to settings/general/restrictions off.
If it does not solve,you can use Apt-get&install Quickgold from the SSH prompt.
Worst case scenario,if nothing works,delete the files located at/var/mobile/Library/Preferences/com.apple.SpringBoard.plist&reboot your phone...