I have a leak in my cold water faucet, and I need to shut off the water until I can fix it. The valve under the sink will not turn with strong hand pressure. How hard can I attempt to turn it before I risk breaking something? (can you tell plumbing is not my area of knowledge? )

I have a leak in my cold water faucet, and I need to shut off the water until I can fix it. The valve under the sink will not turn with strong hand pressure. How hard can I attempt to turn it before I risk breaking something? (can you tell plumbing is not my area of knowledge??)

This valve is called a "angle stop" (see image). When I'm faced with this problem I take a pair of pliers and grip the handle, ( you don't have to grip real tight, you're just using them for leverage)and turn it clockwise to shut it off. Tom