As seilogramp wrote, if you can see recent data, but the old one is missing, then it almost certainly means your device is connected with multiple GC accounts (in other words, you are logged into another GC account than previously). Unfortunatelly it is far too easy to make this mistake, and a big number of people do it all the time for one reason or another, without evn noticing it. Follow the instructions given in the document linked by seilogrampData is Missing from My Garmin Account After Syncing. It may be difficult, though, to merge both accounts now - you will need to export the data from the second account, importing it to the first one.
