losing information

losing information

hi there,

am having problems with data disappearing when I paste it in from Excel. whenever there are more than about 500 lines pasted in they appear to be copied in correctly and saved etc. but then when I go back all the data has gone. is there any reason for this and is there a better way to copy this information in so I won't lose it. I am copying in to an existing worksheet which 3/4 other people have access to. thanks.