0 V at what load? Use 555-3 to drive a reed relay with mercury wetted contacts to give near 0 V up to relays rating. Or use power FET with inversion from 555 to give around 50 m ohms.
The output saturation voltage of the 555 timer's pin 3 will increase with the amount of current you're trying to sink.
At light loads, it's nearly 0v. As you approach 200mA sink current, the saturation voltage increases. If you attempt to sink more than 200mA, the saturation voltage will become excessive.
I was thinking about that while driving. I suspect the OP is trying for more than 0.1A, it seems to be a common mistake. 200ma (the absolute max, you should not try for more than half that) is plenty of current for most applications, if you need more you need transistor drivers.
If you have pushed this chip too hard forcing it beyond its limits it is probably blown. They can only take so much.