Hi ya'll! I'm a beginner in the world of plumbing and have a question.

I turned off the hot and cold water supply under one of my bathroom sinks and it remained off for several weeks. When I turned the water back on, the cold water isnt working. The supply line handle turns, but no cold water comes through. I tried turning off the hot water, and then no water flows into the sink.

I'm not much of a plumber but it seems to me one of a few things:
1. I somehow damaged or broke the water supply line
2. There is something blocking the water supply line

A few notes which might be foolish, but I thought it might help with diagnosis.

Bathroom was added on in 2000, so the plumbing is "new", at least by TOH standards!
Cold water is still flowing to the toilet, sink and tub, so it seems to this beginner that it's a problem of the plumbing under the sink specifically.

How should I start in terms of diagnosing and fixing this problem? Is this a beginner/simple fix, or should I call in a pro?