Please help us with the Version of Lightroom you are using,Make and Model number of your Camera.
Try getting the free Adobe DNG converter and use it to make a copy of your RAW files in DNG format. The DNG format has evolved over time and there are different versions that you can specify in the program's settings. Try converting to the oldest version of DNG format and that should work. The newer versions of DNG just support additional features. It's also a good way to take modern camera files and use them in older programs that read DNG.
DNG Converter Downloads:
How to use DNG Converter