It's a harsh lesson if you made none.
These files are CHK files. They are Lost File Fragments and are parts of files that weren't closed properly when there was a program or system crash.
Normally, if you had just one chk file, or a few small ones, and you knew what they were beforethe check, you could try and give each file their extension back, eg File001.chk becomes File001.mp3, and see if that works.
But if the fragments are large, then decoding becomes problematical. However, if you Google CHK Files, you will find some data recovery options, eg the first one I saw was this;
If you are getting a lot of bad sectors and clusters you should consider backing up all of your data soon, before you lose everything.
Recently I had some problems with my OS in that when I re-booted I began getting the disk check utility running and it found bad clusters in my hard drive. My hard drive is partitioned into a C drive with only the OS and programs on it and a D drive with all my files saved there. The scandisk found bad clusters in both partitions. When I chose the automatically repair option I thought my problems would be fixed. Instead what happened was that it took a folder which had about 200 mp3's in it and converted it into a file with no file extension. So it isn't even recongnized by my OS.
Does anyone no if there is a way to convert this back to the folder with my mp3 files intact?