The drain cover should not have come loose from wear and tear. Some one either put in the wrong screws, did not tighten down the correct screws or did not put in the screws and the plaster held it on for awhile. Its very important to put the cover back on with the correct screws (stainless steel). If the pool water is cold then it looks like its wet suit time. If you do not have a wet suit and diving gear then you could call a pool company (preferably the company that built the pool and do it FOR FREE). Most leak detection companies would do the work also.

Hope this helps