Most times a bathroom sink is clogged with long hair and it’s caught on the drain pop-up lever. If it’s not there, flush out the disconnected p-trap pipe. If you don’t find the clog there, unscrew the last section of pipe on the wall.
If the clog is not there, then it must be stuck very deep in the pipe. You can buy what is called a “Snake” at Home Depot or Lowes and use it to try and get at the clog but it’s a lot more work.