cannot tune in either of the 19" technika tv's i have?

I have 2 tv's which i cannot tune in.I had a tecnika in my daughter's room before which was tunesd in but when it broke i purchased another one and it says no signal the same as the one i have just bought.I have a the aerials going through a booster and had a signal before on my digi box..there is nothing in the manual to help..