Is the crack in the actual screen or in the outer case? Also, is the screen type a CRT (old-style, big box) or a LCD/Plasma type (thin, wall mountable). Also what make/model is it?
In either case make sure the unit is switched off and not used.
If the screen itself is damaged, it will likely need a replacement screen component (which will likely be costly) but may be cheaper than buying a replacement TV.
If you can find the make and model of the tv (model number usually on a sticker on the back of the unit) it should be possible to search on google for TV spares. I've repaired a couple of TVs in the past using this, for not much money.
M. Shaw - Electronics 'A' level grad, BSC Electronics student., April 2008
