Songs have disappeared out of my Itunes library

I am using Windows 8.1 , which is currently upgrading to Windows 10. I have had no problems with Itunes until a couple of days ago. I have 4536 songs in my library and on my IPod. When I now go into my ITunes library it only shows about a quarter of these. I have tried to add another album to my Ipod today and it will not do it. All thoughts would be appreciated

Cheers