On a manual transmission Chevy cobalt

Could be water in the clutch line, flush that and see if it helps. If not see if you're able to engage the gears if you kick the clutch pedal down kickly, if that works it shows that the seals in either the slave or master cylinder are worn and not holding pressure. By kicking the pedal you'll create the needed pressure to engage the clutch. If neither of those work your clutch is probably worn out. Hope this helps