Hard Drive realacating divisions

I am runing win 98 On start up I was confronted with a message that the D drive being the partitioned drve was not readable
Abort,Retry. I entered Abort and windows Opened.
However, when opening explorer the D drive nolong showed and the files that were on the D drive have moved to an new E drive the CD-Rom has become F
drive & the RW has became G drive.
Can someone tell me why this has happend and can I revert back the the original configeration and how