If there are ore blocks among the collapsing blocks, you will lose ore. Otherwise, you just lose stone and clay and sand. Then the only thing you lose is time as you dig up the debris to get back to the ore and shore up your mine (and put in a fake ceiling now)
You will, occasionally, get scrap iron drops (the crossed I-beam block) from falling ore blocks. If I read the xml correctly (blocks.xml, "terrOreIron") it's a 40% chance.