I fell and hit my leg against the cupboard i fell from i applied ice pack immediately but now a small area next to the gash is numb, why?

Unrelated: If you remember that the nerves in the skin run in all directions including across the area that is now your open wound. In addition, there is inflammation of healing (red, hot, swollen and tender) that may involve that nerve. Plus, the trauma may have cut or otherwise damaged that nerve. The good news is that skin nerves usually resolve and return to function depending on the injury severity.

Any injury>nerves: Any cutaneous injury may transect nerves resulting in temporary or permanent nerve injury depending on the extent of the injury. It is not uncommon for nerves surrounding a wound to sustain a bruise like injury and temporarily shut down (neuropraxia). Function tends to return after 3 weeks with a pins and needles type sensation preceding return. The ice could have also been an unlikely cause.