A common mistake in Scheme is to forget the closing parentheses of expressions. If you forget a closing parentheses--usually because you need several to close nested expressions--most systems will just hang, waiting you to finish typing the expression.
This is a feature, not a bug. It lets you put <RETURN>s (line
breaks) in your input, to format the code on the screen as you type it in.
When you type in the last closing expression and hit <RETURN> again,
Scheme recognizes that you've typed in a whole expression, and evaluates
it and prints the result.
So if you type in an expression and hit <RETURN>, and Scheme doesn't
do anything, check to see if you closed all of the parentheses you opened.
If not, just type in the missing parenthesis and hit <RETURN> again.
(It's also possible that in your system, you have to do something
special to get Scheme to evaluate an expression--like hitting a
different key, or clicking on a button or a menu item. In such
systems, <return> may be only for formatting the text you're
inputting, and another key tells Scheme to go ahead and evaluate
what you've typed.)