I have added currency switcher on My Magento cart page, however when I change the currency if there are any products in the cart, Its returning 503 error or the blank page.
Usually, It is caused by following codes from any extensions/theme which will be used/executed on the current page
Mage::getSingleton('checkout/session')->getQuote()The reason is if you added products to the cart and change currency, an infinite loop of getting the quote items will be executed.
Try to find the above codes from your extensions or theme and replace with the following codes
$quoteId = Mage::getSingleton('checkout/session')->getQuoteId();
$quote = Mage::getModel('sales/quote')->load($quoteId);