Sometimes during combat Eldritch Blast disappears. I'll cast Hex and then click on Eldritch Blast and instead of casting, it disappears from the tool bar and I'm not able to cast it again until after a long rest. I remember this happening with other spells/attacks with other classes, but it doesn't happen often, so I'm not sure which they were. I know it did happen with Gale's Ray of Frost once or twice. It's super annoying.
BTW this is different from when a spell can only be cast once or twice a day and then goes grey, these are cantrips, right? And even if they weren't they shouldn't completely disappear from the bar, they would go grey and unselectable.