I installed openwrt on linksys e4200 v2 years ago, but didn't use it often. Today I started it, and found the router back to the linksys factory firmware itself !!!, don't know what happened. Does this mean my openwrt is gone whatever?
Like many other (newer) Linksys device, yours probably has a dual-boot setup, which toggles the bootflag after three incomplete/ 'failed' boots - see e.g. the wrt3200acm wiki page as example. You can probably switch back to the other partition with OpenWrt by faking three failed boots.