Sorry to read you are having problems that's quite a drop in eggs. There are several things that will put birds of lay, a few more details may help someone give you an idea what it might be.
How old are the birds and were they inoculated? Are they in an enclosed run or free to wander? Have they had a change of feed, water accidently spilt so they have gone short? Been kept in the dark for long periods? How's there face colour and combs? Any upset and fright?
Also anything unusually like sneezing, head shaking, discharge from the nostrils, poo changed, loosing feathers, eggs pale or misshapen?