What would happen if a 13a fuse is put in a tv?

Nothing until the tv develops a fault, then it might catch fire and set your house alight. The correct value of fuse should always be used.

A fuse is a circuit safety device. Asking this question must mean that the fuse has blown. It has blown for a reason. The reason has to be found before a new fuse is installed. If the reason has been found and corrected then the fuse that the manufacturer recommended should be re installed. It is your option to use a larger fuse that what is recommended. On a fault condition by not limiting the current to a certain amperage there is the chance that more damage to that circuit's components will occur and irreparable damage to the TV could make it useless.