Can you go in windows?
If yes you could try the On-Screen Keyboard accessibility tool.
You could try some windows 98/95 bootdisks (on floppies), boot from them and in the console wait few seconds: if it's a letter it may be appear in the console.
Try unplugging and plugging back in the keyboard. Is it a PS/2 or USB?
If is PS/2 check the connection for damaged pins