clearly you didnt put the belts back on the same way they came off
pull the deck off and double check the belts are on all the pulleys and in the right direction on the idler pulleys
then release the brake and crawl under and the belt should be tight
did you drop the crankshaft pulley to replace the drive belt? if so its really easy for it to slip off when reinstalling that pulley because of the way its surrounded by retaining tabs
