If required yield in the economy increases because of macro monetary influences, the influence of inflation or yield spreads widening, existing bonds will decrease in price and this is because if a bond is trading at 100 and has a coupon of 5% and an increase in inflation by 1% increases the required yield by 1% then a comperable bond can be issued at 6% and trade at par.
What happens to existing 5% bonds? Well you definatly can't sell it at par because you could buy a 6% bond at the same price, so you can only sell it for less, this keeps yields in line with the going yield in the bond market for that given type of bond.