I ran into a similar experience. I was climbing with a 60%+ win rate and then this past weekend I went 1-7 and since then every game seems like a struggle.
The first thing I did was analyze my own play. Was my cs/min lower? Was I warding less? Was my damage to champions down? You have to be really self critical before you consider any other possibilities.
The other thing to look at is changes in your champion pool. Did you play a lot of one champion and lose with him? Might be time to bench that champion from ranked and practice him a bit more in normals.
The other possibility is your MMR might have gotten so high by consistently climbing that now the system is matching you up with much worse allies. I definitely noticed this. As soon as I started winning a lot I was getting matched up with allies who had like a 30% win rate.
It's frustrating but eventually it evens out. Just be completely honest with yourself and be very self critical of all your mistakes and you'll eventually start climbing again.