Stop panicking. If someone had found a hole in Bitcoin security they would have exploited it with far larger wallets. Just move whatever funds you have to another wallet an get on with your life. If they don't have control your wallet then by deduction they must have the private key to that address. There may have been a stuff up with Blockchain giving you an already used address but the probability is basically 0.
It's NOT ZERO
it's 1/115792089237316195423570985008687907852837564279074904382605163141518161494336
and totally NOT impossible to guess the key pair.
Please read the bolded part again - basically 0. I'm not saying it can't happen, but this is still a supremely unlikely occurrence to the point I'm far more inclined to believe some alternate explanation.
The term is infeasible. Then again just about anything which most people consider is impossible is simply low probability. Trying running directly into a wall. There is a non zero percent chance that due to quantum effect you will pass right through it.