In Scene 7 of "The Glass Menagerie," there is a power failure at the beginning of the play. What does that mean both literally and figuratively?

In Scene 7, the power goes off because Tom has not paid the bill, having used the money to join the Union of Merchant Seamen.  With the electricity gone, candles are used to light the room, lending shadows, thus allowing reality to be transformed into illusion, which is the purpose of Expressionistic plays such as "The Glass Menagerie."  In the darkened corners of the room, shy Laura hides; however, Jim brings her out of these shadows by literally asking her to move, and by figuratively giving her some confidence in herself.

Light plays an important role in this final scene.  It lends an unearthly beauty to Laura, bringing out her fragile nature, and it suggests illusion.  Her virginal presence in white with the candles, suggests a religious ceremony.  Tennessee Williams's stage directions mention this symbolism:

JIM lights a cigarette and leans indolently back...smiling at LAURA with a warmth and charm which lights her inwardly with altar candles.

Over the light Jim handles the glass unicorn.  Laura tells Jim, "You see how the light shines through him?"  When they dance, Jim tells Laura "I'm not made of glass."  But, they bump into the table, knocking off the unicorn whose horn breaks.   Now, the unicorn is not odd; like Laura at this moment, he is normal.

As Jim and Laura sit and talk, they sip lemonade brought by Amanda, who spills it, saying, "Ooo! I'm baptizing myself!"  The religious symbolism continues with the lighted candles.  When Jim reveals that he is to be married, the stage directions make allude to this symbolism:

The holy candles in the altar of LAURA's face have been snuffed out.  There is a look of almost infinite desolation.

Then, while Tom as narrator looks in at Laura, he tells Laura that no matter how far he has gone, he cannot forget her; he runs to the movies, he drinks in order to "blow your candles out!" Tom tells her "Blow out your candles, Laura--and sogoodby..." She bends over the candles, ending the play.  The illusion is finished.