Reinforced Shimano pin snapped before connecting chain - how to resolve?
You can often use the chain tool to drive the half-way inserted pin back out from the chain. I do not think it is advisable to attempt pressing it in again. It is certainly possible, I did re-pressed regular roller pins a couple of times when I or someone else accidentally pressed them out too far so that they fell off. It is not the most enjoyable experience, especially as the roadside repair.

You can get another reinforced pin and try again. Alternatively, you could get a so-called master link which replaces one of the half-links on the chain (presumably one at the end of your chain with the broken pin sticking out of it). The master link allows to close the chain using no tools at all, and often to open the chain using special pliers (or just bare hands, with enough swearing).



Several companies manufacture such links for chains of different widths (for cassettes from 8 to 12 speeds). They are generally compatible with chains of all vendors, e.g. you can use a KMC master link on a Shimano or SRAM chain with no issues.