I know that 冬ごもり means "winter solitude" so it's clear what role 冬 has in that, but what does ごもり come from? I looked up "solitude" and "isolation" on jisho but nothing with that pronunciation seemed to come up. Maybe I'm looking in the wrong place.
It is normally a good idea to have a few different dictionaries you can refer to. In this case, however, it would be difficult without first removing the 濁点 (dakuten) and changing こもり to the standard verb form こもる. If you do that and try various readings, 籠る will give you this page, definition 1 being pertinent:
to shut oneself in (e.g. one's room); to be confined in; to seclude oneself; to hide away; to stay inside (one's shell)
Incidentally, the kanji for 籠る can also be written 篭る, although hiragana is more likely to be used (e.g. 引きこもり).