What is the value of a 1776-1976 silver US half dollar?

If your coin has no letter above the dual date, or a small D, it's made of copper-nickel and not silver. The last circulating silver halves were dated 1969.

If it has an S it's a proof coin and may be copper-nickel or 40% silver, but these coins weren't put into circulation. Check the edge to see if it's copper in color.

There's more information at the Related Question.
None of the bicentennial coins made for general circulation contain any silver or are worth more than face value. Only Proof and collectors coins sold from the US Mint are worth more.