CD-ROM Drive Letter

Some how the drive letters on my PC have been changed and my DVD/CD-ROM Drive D:\ has been changed to a “Removal Storage Drive” device. I have an External Zip Drive as E:\ drive. I also have an and internal CD read/write re-write, burner on drive F:\. I can install new software, play audio CD’s, run software or copy CD’s until I fix this.
Any suggestions or tips would be appreciated.
Thanks,

Paul