single fish suddenly died

Hello,

I had an small female Lyretail Anthias sudden die. She was in my 55 gallon tank. I had her for several months. A couple of days ago I noticed she hid a lot which she normally didn't do during lights on time. Tonight I found her dead. I inspected her closely and found no spots, fin rot or damage or any imperfections at all. all other fish and inverts in the tank are fine. I tested the water and it tested ok (1.025sg, 8.2 ph no ammonia or nitrite, less than 5.0 ppm nitrate, 8dkh hardness, 0.01 phosphate) the only problem I found was temperature heater wasn't working apparently and the temperature fell to 72 ( it's normally 78) could this do it??? Any other ideas???

Thanks,

Kyle