Rank: Wiz
Rating: 86%, 60 votes
I believe there is a battery backup for the memory & it may have run down. I don't know if it's a consumer or factory fix. Consult the owner's manual. If you don't have the manual, copy & paste the URL I've provided to your browser & that will take you to the 2820 manuals page. The quick reference & owner's manual are available free as downloads. Good Luck.
https://buy.garmin.com/shop/store/manual.jsp?product=010-00517-05&cID=134&pID=382