You have three options:
1. wait for it to appear again (might take a while)
2. Go to Settings > ABOUT TABLET > then choose and click FIRMWARE UPDATE
3. You can go to the archos website, go to downloads and click frimware (sorry i can not post here the exact link, forum says i am not all0wed)
then download the firmware on your PC, then upload it on your tablet. Follow the instruction on that page.
#3 option worked for me
MigsMobile dot net