Yep! .....been there as well. I describe this as "hitting a wall".
Once you manage to push through it, the rest of the night usually goes fine.
I keep a selection of fairly generic 20 minute mixes. They normally live on my iPad or on CD for backup purposes, but i've also got them in my DJing library as well. If I've hit the wall & i'm really struggling, i'll stick one of these on, grab a drink & just stand back for 10 minutes.
I'll also probably do this with my headphones on, but nothing playing through them. ....just to give my ears a rest as well.