A Morgan dollar without a mint mark would have been minted in Philadelphia. The P mint mark wasn't used on $1 coins until 1979.
But more information is needed to determine a value. Morgan dollars were minted from 1878 to 1904 and again in 1921 so you need to know the coin's date, plus have an idea of how worn it is. Please post a brand-new question with the coin's date, or enter the question "What is the value of a [date] US silver dollar?" in the box at the top of the page. [date] is of course your coin's specific year.
$15 to $20 depending on condition and variety.
Post new question with The Mintmark please.
$15-17 retail for a coin in average condition.
$27 to $1300 depending upon the condition of the coin.
The 1897-S MORGAN dollar is not scarce or rare, an average retail value for the coin is $26.00-$32.00.
As of 09/2009 the melt value of a Morgan dollar is about $8 to $10. However its collector value can be much higher depending on its condition and mint mark.
No such thing as a "strike mark" but U.S. coins do have Mintmarks. For Morgan dollars, they are on the reverse just above the DO in DOLLAR.
The 1885-O Morgan dollar is common, in general values are $32.00-$38.00 depending on grade.
The mint mark on any Morgan dollar is on the reverse above the letters DO in dollar.
1921 Morgan dollars unless they have an exceptional strike and are virtually perfect generally sell for about their silver melt value, about $25 at current melt value.
About $17 or $18, or more if silver rises dramatically in price (as of January 30, 2008, silver is $16.76 per troy ounce and the Morgan silver dollar has an Actual Silver Weight of 0.7734 troy ounces, giving it a "melt value" of about $12.96). Note that there would be no "P" mint mark - coins produced in Philadelphia at that time had no mint mark.
If it has one, it's on the reverse above the DO in DOLLAR.