。
I don't know what will happen next year.
Literal
Next-year what-kind-of [attributive-な] thing [subject-が] happen [nominalizer-のか] understand-not.
どんな事 ('what kind of thing') + 起こる ('happen') makes an embedded question. のか = の (nominalizer/explanatory) + か (question), marking the embedded interrogative. 解らない is the kanji spelling of わからない ('don't understand'). The 解 variant emphasizes comprehension, distinct from 分かる (distinguishing/knowing).