An umbilical hernia is not dangerous in itself, but there is a risk that it will get trapped (incarcerated). This can cut off the blood supply to the contents of the hernia, causing life-threatening conditions such as gangrene or peritonitis (if this happens, the hernia is said to be strangulated).
https://global.ihi.com/alarm+service/health+fact+sheets/umbilical+hernia+repair+in+adults.aspx