The strap is broken on casio watch
I had exactly the same problem with exactly the same model watch, and it happened to me twice. The AQ-160 is the important part of the model number as far as replacement parts are concerned. The first time, I took it to the shop where I bought the watch, and they replaced the band, but it cost me $50. The second time, I looked up the Web for the master agent for Casio in Australia, and they said I could buy the end piece, not the whole band, for $6. I found I could replace it quite easily myself by using a drawing pin to remove the spring pin that holds it together. A bit fiddly to reassemble but not too bad. In fact, I didn't wait for the new part to arrive, just used the unbroken end of the old band. So now I have a repaired band plus the old band and a new end piece, in case it ever happens again.
I really like this watch, but it is a pity the bands are so weak.