Cracks in the teeth appear all the time. Depending on how deep the crack, how extensive the area, and how clean the area is this can range anywhere from a problem that may get worse to an imminent looming threat. It is definitely an occasion to go and book an appointment to your dentist.
Why did my tooth crack?
This is an answer that only your dentist can truly answer, and only after a proper diagnosis of the situation. Usually, teeth either crack because of a sudden change in temperature, because of trauma to the teeth, or because the tooth is dead or extremely demineralised. Sudden changes in temperature can cause rapid expansion or contraction of the tooth enamel and the tooth structures, and this can cause cracks to form, and can even crack the entirety of the tooth. Trauma to the teeth can also crack them, obviously. The tooth has natural defences against cracking and can repair itself to a certain extent, but once the tooth is dead, it does not do so. This is why teeth that have been root canaled, or teeth that have already been filled or have had work done on them are so likely to crack: because they are dead and do not repair any more.
What can happen
Apart from becoming sensitive almost immediately, these teeth will start to hurt, unless they hurt already. Although dead teeth are not connected to the nerve, the gums can become inflamed around it, putting pressure on the dental nerve and becoming painful over time. The crack in the tooth will start to fester, and the bacteria now have free reign to go into the internal structures of the tooth, where they can do untold damage. Very frequently, these cracks go along the length of the tooth, and thus provide access to the nerves easily, which means they can become extremely painful.
What you can expect
If you go in time, the tooth can be treated with just some fluoride ointment put on the crack, which will seal up. If the crack is wider, than you can use a little bit of composite resin to seal the crack and secure the tooth. If the tooth is already dead, then either a root canal or an extraction may become necessary.