restore a library backup -- you should have one done automatically. Ah .. see someone else is quicker
What might have happened is that there were external changes to your file directory structure (or an external program like Tag/Rename).
If last played is very important to you, you can go to Options=>Library&Folders=>Manage Library Fields and check the box write to file if possible (it won't write a date but will write a numeric string that can be then reintegrated into the library from tools. This works well with flac files ... some file formats will not support this, for info. You can check by doing a tag dump (see the wiki) to see what tags are actually embedded in the file itself.
The downside of this is that if you do incremental backups of your media, you will have to backup all files you have played since the previous backup.
Note: one field you can't do this for is date imported
Note 2: one program (makeMKV) I have does something to files for some reason even though I have not processed any videos through it. After opening a folder, JRiver does reimport the whole root directory .. strange - I'll look into that one