When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with C-g, which causes a quit. See Quitting.
Ordinary quitting gives no information about why the program was
looping. To get more information, you can set the variable
debug-on-quit to non-nil. Once you have the debugger
running in the middle of the infinite loop, you can proceed from the
debugger using the stepping commands. If you step through the entire
loop, you may get enough information to solve the problem.
Quitting with C-g is not considered an error, and
debug-on-error has no effect on the handling of C-g.
Likewise, debug-on-quit has no effect on errors.
This variable determines whether the debugger is called when
quit is signaled and not handled. If debug-on-quit is
non-nil, then the debugger is called whenever you quit (that
is, type C-g). If debug-on-quit is nil (the
default), then the debugger is not called when you quit.