scenarioA: if you made an sd-ext partition for your sdcard, you would need to remove this by saving the data on your card to the computer, and then reformatting the sdcard. once formatted you can copy the data back to your sdcard.
scenarioB:if you did nothing to your sdcard to make you think you have a sd-ext, then we are gonna need more info.
scenarioC:save any info app data info you wish to keep(for example:angrybirds), reboot to recovery, and reflash your rom.
and I have to ask, how old is your sdcard and what is the class lvl?