on the face of it it looks OK
so either the select isn't finding any rows for the specified values
there is something odd going on with MySQL
my first guess would be the '$table', php will replace the value of $table, so in effect your query is reading
"SELECT pwd FROM `_lisausers`....
next would be the where clause
are you certain that $mypassword contains a value that is in the db.
If SQL aint doing what I expect, I always want to see the SQL that is actually being sent tot he server, it may not be what you think it is. If the SQL looks ok then I like to place the same SQL into a query browser just to verify its legit
one way of checking the basic functioning of the logic is to temporarily replace the SQL in question with something that is known to work, ie without a "where clause".
is the password encyrpted in the db (ideally it should be, otherwise you have the same weakness as no password, ie the user can see their password). if so are you certain that you are using the same encryption method in both cases. it can be difficult to manually do a comparison between encrypted passwords.
That should give you some results. If this is the case then as healdem suggests it's probably a problem with your where clause, your column selection, or the possibility that you have encrypted passwords in the table??
Either way put in a "echo mysql_error();" at the bottom of your script to see if you're getting any errors.