I have a mac computer. The little icon "Macintosh HD" got removed from the main page. My comuter doesn't

act right. Can someone please tell me what to do! I tried dragging it back to the main page but it doesn't work. HELP! Before this, everything started flashing, the applications would be shown and then disappear and they would do that constantly. HELP HELP HELP!! It's not an old computer. It's less than 1 year.