If you recently installed a new tweak or extension in Cydia, try uninstalling that tweak. Open up Cydia, go to Installed, tap Recent, find the tweak and tap it, tap Modify in the top-right corner, and tap Remove to uninstall it.
Especially try uninstalling any extensions that may not be compatible with your device's iOS version (often the description page for an extension mentions which iOS versions it is compatible with). Also make sure you're using extensions from trusted repositories such as BigBoss, ModMyi, and ZodTTD & MacCiti.
If you haven't recently installed an extension, or uninstalling it didn't help, read on.