OK.
I assume the dead HDD is the one in POS that the cash register is attached. This is also your server.
Assuming that you have a backed up database...
Reinstall from RMS original disk the SQL server and when prompted supply the server password. This is not the same as your database. When asked click mixed connecting mode, that is connect to server via Windows or Server authentication.
Reinstall RMS and supply the key that is on the disk or documentation if asked.
Open RMS Administrator and connect via server authentication (may be SQL authentication). you need to provide the correct server name on top (click the drop down button to see what is available), and databse file Master.mdf, connect.
Once connected, go to Select > Create database and follow the prompts to restore your back up.
Once restored, select this new database and go Configuration > Test connection (here supply the server name and new file name correctly).
If connection is successful close Administrator and open Manager. Now you should be able to see your old data. Then you have to run POS here you will need to talk to Microsoft to give a verification key if you don't have it. You may need to reset paths, balance, register what ever attached to it.
Then you have to check your network and connection to server so that the back office can connect.
Good luck