MOST of my students, and myself - twice - have had cello tuning pegs let go with the recent cold spell. Because the pegs and pegbox are constructed of different woods, they shrink at different rates with changes in humidity and temperature. This causes the peg to lose its pressed fit and pop! ... the string is loose. In many cases, the string needs to be rewound from square one.
Click here for your tutorial in how to properly wind your strings! Book mark the page for the occasion of new strings, too!