If the product is from the old CP store (the one that pre-dates SMS take over), you had 30 days to download your order history before SMS deactivated the links.  You can still access the order history (mainly to check if you own a product) but SMS will not activate the links for any of these products. 

If the products were purchased after the SMS, you several options.  You can purchase the extended download service which gives you two years of active links.  If you purchased EDS and two years have passed, you can repurchase the EDS and gain another two years of active links.  If you did not purchase the EDS at the time of purchase, you have download the product within 30 days of purchase as that is the length of time SMS keeps the link active.  You can purchase the EDS at a later date if you did not originally purchase it.  EDS is based on price up to a set amount.  If the order's amount exceeds the set amount, you are charged the max which is a little under $9.