It makes it 340282366920938463463374607431768211456 times easier to brute force.
So if you could guess 10^9 per second it'd only take you 10^24 years instead of 10^63 years.
256 bit ECDSA keys only have 128 bit security. Half of an ECDSA key would be 64 bit security. While a naive attack would be to increment all possible private keys there are more sophisticated attacks (
https://en.wikipedia.org/wiki/Pollard%27s_rho_algorithm_for_logarithms ) which are of complexity O( n^(1/2) ) steps where n is the key length.
64 bit security would be breakable but it is very likely the cost to break they key would be greater than the reward. Although if this isn't a hypothetical I would recommend transferring the coins now.