As Ahmed said, you seem to have defined y[0]. This is usually done by accident using a single = instead of a double == in an earlier code. You can restart the kernel or try y[0] =. or Clear[y].
It's trickier when one makes this mistake on the derivative equation. To clear y'[x], you use Clear[Derivative] or y'[x] =. (this does not seem necessary in the OP's case, though.)
