That's because your champion roster is full - the 40/40 right below the big i shows that. So you can't summon any more champions unless you make space, by either sacrificing some champions, moving some to the vault or increasing your roster space (using silver or gems). 

Or by upgrading your champions roster and/or Vault capacity.

I am having the same problem, but my roster shows 390/400.

I should have mentioned that this is only happening in the new "Summon Pool".

I believe there is currently a bug where if you try and summon without enough crystals it freezes.  All accounts I've heard of closing the game fixes it without any other ill effects.