Hi, My VM 3 site had this problem today. I could not find much info online, yet a post mentioned to uncheck the "Display modal popup upon 'Add to cart'" option on Checkout tab of configuration (first option) which when I did reported that I had a duplicate slug in my _virtuemart_products_en_gb table. I fixed this and the VM 3 started loadin products into the cart again.
So, if you have this issue you might want to check out that your database has no duplicates.
Mati
Read this
http://forum.virtuemart.net/index.php?topic=79799.0 (http://forum.virtuemart.net/index.php?topic=79799.0)
regards
Jörgen @ Kreativ Fotografi